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PubMed2006

Automatic identification and classification of characteristic kinetic curves of breast lesions on DCE-MRI.

Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) of the breast is being used increasingly in the detection and diagnosis of breast cancer as a complementary modality to mammography and sonography. Although the potential diagnostic value of kinetic curves in DCE-MRI is established, the method for generating kinetic curves is not standardized. The inherent reason that curve identification is needed is that the uptake of contrast agent in a breast lesion is often heterogeneous, especially in malignant lesions. It is accepted that manual region of interest selection in 4D breast magnetic resonance (MR) images to generate the kinetic curve is a time-consuming process and suffers from significant inter- and intraobserver variability. We investigated and developed a fuzzy c-means (FCM) clustering-based technique for automatically identifying characteristic kinetic curves from breast lesions in DCE-MRI of the breast. Dynamic contrast-enhanced MR images were obtained using a T1-weighted 3D spoiled gradient echo sequence with Gd-DTPA dose of 0.2 mmol/kg and temporal resolution of 69 s. FCM clustering was applied to automatically partition the signal-time curves in a segmented 3D breast lesion into a number of classes (i.e., prototypic curves). The prototypic curve with the highest initial enhancement was selected as the representative characteristic kinetic curve (CKC) of the lesion. Four features were then extracted from each characteristic kinetic curve to depict the maximum contrast enhancement, time to peak, uptake rate, and washout rate of the lesion kinetics. The performance of the kinetic features in the task of distinguishing between benign and malignant lesions was assessed by receiver operating characteristic analysis. With a database of 121 breast lesions (77 malignant and 44 benign cases), the classification performance of the FCM-identified CKCs was found to be better than that from the curves obtained by averaging over the entire lesion and similar to kinetic curves generated from regions drawn within the lesion by a radiologist experienced in breast MRI.

Adult↗

PubMed2003

Kinetic analyses of three distinct potassium conductances in ventral cochlear nucleus neurons.

Neurons in the ventral cochlear nucleus (VCN) express three distinct K+ currents that differ in their voltage and time dependence, and in their inactivation behavior. In the present study, we quantitatively analyze the voltage-dependent kinetics of these three currents to gain further insight into how they regulate the discharge patterns of VCN neurons and to provide supporting data for the identification of their channel components. We find the transient A-type K+ current (IA) exhibits fourth-order activation kinetics (a4), and inactivates with one or two time constants. A second inactivation rate (leading to an a4bc kinetic description) is required to explain its recovery from inactivation. The dendrotoxin-sensitive low-threshold K+ current (ILT) also activates with fourth-order kinetics (w4) but shows slower, incomplete inactivation. The high-threshold K+ current (IHT) appears to consist of two kinetically distinct components (n2 + p). The first component activates approximately 10 mV positive to the second and has second-order kinetics. The second component activates with first-order kinetics. These two components also contribute to two kinetically distinct currents upon deactivation. The kinetic behavior of IHT was indistinguishable amongst cell types, suggesting the current is mediated by the same K+ channels amongst VCN neurons. Together these results provide a basis for more realistic modeling of VCN neurons, and provide clues regarding the molecular basis of the three K+ currents.

Animals↗

PubMed1999

Effects of exercise training on oxygen uptake kinetic responses in women with type 2 diabetes.

OBJECTIVE: Women with uncomplicated type 2 diabetes have both a decreased maximal oxygen consumption (VO2max) and slowed oxygen uptake (VO2) kinetics at the onset of exercise compared with nondiabetic women. These abnormalities are seen not only at maximal workloads, but also at the onset of low-level exercise. To evaluate the hypothesis that VO2max and VO2 kinetics would improve with exercise training in untrained people with type 2 diabetes, we measured these parameters in premenopausal sedentary women before and after 3 months of supervised exercise training. RESEARCH DESIGN AND METHODS: A total of 8 women with type 2 diabetes, 9 overweight nondiabetic women, and 10 lean nondiabetic women were studied. At baseline and after 3 months of exercise training, subjects underwent bicycle ergometer testing to obtain VO2max and VO2 kinetics data. RESULTS: On entry, women with type 2 diabetes had the lowest VO2max and slowest VO2 kinetics of the three groups. After exercise training, the women with type 2 diabetes improved their VO2max more than the lean and overweight control women: 28 vs. 5 and 8%, respectively (P < 0.05 for the diabetic group vs. both control groups). In the group with diabetes, VO2 kinetics improved by 39 and 22% at 20 and 30 W, respectively. For the control subjects, VO2 kinetics did not improve at any workload in either group. CONCLUSIONS: Despite beginning with the lowest VO2max and slowest VO2 kinetics, subjects with type 2 diabetes benefited more from an exercise training program than did control subjects. These findings suggest that in addition to its known metabolic effects, exercise training in individuals with type 2 diabetes may be an effective therapy to improve the cardiovascular response to exercise and to overcome low-level exercise impairment as reflected by improved VO2max and VO2 kinetics. If the ability to make circulatory adjustments at the beginning of exercise at low workloads is improved by an exercise training program, as suggested by the VO2 kinetics data, the clinical significance of exercise for people with type 2 diabetes is clear.

Adult↗

PubMed1985

Analysis of ligand binding to receptors by competition kinetics. Application to muscarinic antagonists in rat brain cortex.

We propose a method for obtaining kinetic parameters of unlabeled ligands which is based on analyzing the effect of their competition on the binding kinetics of a labeled ligand. In this method (competition kinetics), the binding kinetics of a labeled ligand are measured in the absence and in the presence of given concentrations of a competing unlabeled ligand. The rate equations appropriate to the particular kinetic model may be solved employing linear homogeneous differential equations. The rate constants appearing in the final integrated expressions can be evaluated by nonlinear regression. We have demonstrated the validity and applicability of the method to study receptor systems, using the muscarinic receptors in rat brain cortex as an experimental system. We have determined the various kinetic parameters of two muscarinic antagonists (N-methyl-4-[3H]piperidyl benzilate and (-)-N-[3H]methylscopolamine) by following directly their binding kinetics. These parameters were compared with those obtained for the same unlabeled ligands in homogeneous and heterogeneous competition studies. In all cases, the parameters obtained by the competition kinetics method were close to those extracted from direct binding kinetics, and in cases where both types of studies were performed on the same preparations, the parameters obtained by the two methods were essentially identical.

Animals↗

PubMed2002

Thermodynamics and kinetics of protein folding: an evolutionary perspective.

This article appeals to an evolutionary model which postulates that primordial proteins were described by small polypeptide chains which (i) lack disulfide bridges, and (ii) display slow folding rates with multi-state kinetics, to determine relations between structural properties of proteins and their folding kinetics. We parameterize the energy landscape of proteins in terms of thermodynamic activation variables. The model studies evolutionary changes in these thermodynamic parameters, and we invoke relations between these activation variables and structural properties of the protein to predict the following correspondence between protein structure and folding kinetics. 1. Proteins with inter- and intra-chain disulfide bridges: large variability in both folding rates and stability of intermediates, multi-state kinetics. 2. Proteins which lack inter and intra-chain disulfide bridges. 2.1 Single-domain chains: fast folding rates; unstable intermediates; two-state kinetics. 2.2 Multi-domain monomers: intermediate rates; metastable intermediates; multi-state kinetics. 2.3 Multi-domain oligomers: slow rates; metastable intermediates; multi-state kinetics. The evolutionary model thus provides a kinetic characterization of one important subfamily of proteins which we describe by the following properties: Folding dynamics of single-domain proteins which lack disulfide bridges are described by two-state kinetics. Folding rate of this class of proteins is positively correlated with the thermodynamic stability of the folded state.

Disulfides↗

PubMed2006

Model-free deconvolution of femtosecond kinetic data.

Though shorter laser pulses can also be produced, pulses of the 100 fs range are typically used in femtosecond kinetic measurements, which are comparable to characteristic times of the studied processes, making detection of the kinetic response functions inevitably distorted by convolution with the pulses applied. A description of this convolution in terms of experiments and measurable signals is given, followed by a detailed discussion of a large number of available methods to solve the convolution equation to get the undistorted kinetic signal, without any presupposed kinetic or photophysical model of the underlying processes. A thorough numerical test of several deconvolution methods is described, and two iterative time-domain methods (Bayesian and Jansson deconvolution) along with two inverse filtering frequency-domain methods (adaptive Wiener filtering and regularization) are suggested to use for the deconvolution of experimental femtosecond kinetic data sets. Adaptation of these methods to typical kinetic curve shapes is described in detail. We find that the model-free deconvolution gives satisfactory results compared to the classical "reconvolution" method where the knowledge of the kinetic and photophysical mechanism is necessary to perform the deconvolution. In addition, a model-free deconvolution followed by a statistical inference of the parameters of a model function gives less biased results for the relevant parameters of the model than simple reconvolution. We have also analyzed real-life experimental data and found that the model-free deconvolution methods can be successfully used to get undistorted kinetic curves in that case as well. A graphical computer program to perform deconvolution via inverse filtering and additional noise filters is also provided as Supporting Information. Though deconvolution methods described here were optimized for femtosecond kinetic measurements, they can be used for any kind of convolved data where measured experimental shapes are similar.

Journal Article↗

PubMed1996

A descriptive tool to characterize nonlinear kinetics, with applications to meperidine and lidocaine.

We describe an explorative data analysis tool which can detect and describe the presence of nonlinearities in multiple dose kinetics studies. The method is nonparametric (i.e. not dependent on modeling assumptions), uses regression to estimate the functions representing the kinetics, and makes the detection of nonlinearity a part of the model selection process. Flexible functions (splines) are used to describe the kinetics corresponding to the lowest given dose, and to describe the (possible) departure of the kinetics corresponding to higher doses from the reference kinetics. The estimated kinetics and departures can be examined to offer possible insight into the nature of the nonlinearity. The methodology is applied to the analysis of meperidine and lidocaine kinetics through the lungs and the heart. We find that the lung kinetics of lidocaine and meperidine are linear. However their myocardial kinetics are complex and nonlinear.

Animals↗

PubMed2000

A new in vitro/in vivo kinetic correlation method for nitrofurantoin matrix tablet formulations.

The kinetic distributions of in vitro percentage release and in vivo percentage urinary excretion rates of nitrofurantoin from matrix tablets were plotted using a kinetic program. In vitro release rates were determined using the USP paddle and half-change methods. Urinary excretion curves of the drug were characterized by means of the statistical moments. The individual linear correlations between each in vitro and in vivo kinetic distribution were established, and regression equations were calculated. The application results of the best correlations obtained were evaluated according to in vivo results. A reversed kinetic procedure was applied for transformation of the correlated kinetic values to the drug percentage release rates. The modified Langenbucher kinetic showed excellent linear correlation (r = .9985). The method that is proposed in this study, the kinetic correlation program, is simple, independent of time, and suggests that it is possible to use kinetic distributions in the in vitro/in vivo correlation. This study also suggests using kinetic correlation to investigate the suitability of the in vitro dissolution methods with the in vivo drug dissolution.

Anti-Infective Agents, Urinary↗

PubMed1986

Microcomputer simulation of steady-state enzyme kinetics for educational purposes.

A BASIC program to assist the instruction of steady-state enzyme kinetics has been developed for the IBM PC microcomputer. Its purpose is to simulate laboratory experiments in order to minimize the time required to obtain kinetic data from which students deduce kinetic mechanisms and determine kinetic constants of enzyme-catalyzed reactions. The program randomly selects a kinetic scheme from various sequential, ping pong, and iso reaction sequences as well as values for the kinetic constants. The scheme and kinetic constants are unknown to the student at this time; the only thing he or she knows is the stoichiometry of the catalyzed reaction which can have two or three substrates and products. The student is prompted to enter values for concentrations of substrates and products; several different concentrations for each substrate and product can be entered in a single experiment. The program then calculates, displays and prints (if desired) the corresponding initial steady-state velocities. The student can perform as many experiments as desired until enough information is obtained to determine the kinetic mechanism and to calculate values for the kinetic constants.

Algorithms↗

PubMed2005

Association between subjective lameness grade and kinetic gait parameters in horses with experimentally induced forelimb lameness.

OBJECTIVE: To evaluate the association between subjective lameness grades and kinetic gait parameters and assess the variability in kinetic parameters in horses with experimentally induced forelimb lameness. ANIMALS: 32 horses. PROCEDURES: Forelimb lameness was induced in each horse via injection of lipopolysaccharide into 1 metacarpophalangeal joint (40 experimental trials). Subjective lameness grading and 13 kinetic gait parameters (force plate analysis) were assessed before (baseline) and at 12, 18, and 24 hours after lipopolysaccharide injection. While horses were trotting, kinetic gait analysis was performed for 8 valid repetitions at each time point. Repeated-measures analyses were performed with 8 repetitions for each kinetic parameter as the outcome, and lameness grades, time points after lipopolysaccharide injection, and repetition order as explanatory variables. Sensitivity and specificity of kinetic parameters for classification of horses as sound or lame (in relation to subjective lameness scores) were calculated. Between- and within-horse variabilities of the 13 kinetic parameters were assessed by calculation of coefficients of variation. RESULTS: Subjective lameness grades were significantly associated with most of the kinetic parameters. Vertical force peak and impulse had the lowest between- and within-horse coefficients of variation and the highest correlations with subjective lameness grade. Vertical force peak had the highest sensitivity and specificity for lameness classification. Vertical force peak and impulse were significantly decreased even in horses with mild or unobservable lameness. CONCLUSIONS AND CLINICAL RELEVANCE: Among the kinetic gait parameters, vertical force peak and impulse had the best potential to reflect lameness severity and identify subclinical forelimb gait abnormalities.

Animals↗

PubMed2003

Clonal expansion of adult rat hepatic stem cell lines by suppression of asymmetric cell kinetics (SACK).

Adult stem cells have potential use for several biomedical applications, including cell replacement therapy, gene therapy, and tissue engineering. However, such applications have been limited due to difficulties encountered in expanding functional adult stem cells. We have developed a new approach to the problem of adult stem cell expansion based on the suppression of asymmetric cell kinetics (SACK). We postulated that asymmetric cell kinetics, required for adult stem cell function, were a major barrier to their expansion in culture. As such, conversion of adult stem cells from asymmetric cell kinetics to symmetric cell kinetics would promote their exponential expansion and longterm propagation in culture. The purine nucleoside xanthosine (Xs), which promotes guanine ribonucleotide biosynthesis, can be used to reversibly convert cells from asymmetric cell kinetics to symmetric cell kinetics. We used Xs supplementation to derive clonal epithelial cell lines from adult rat liver that have properties of adult hepatic stem cells. The properties of two Xs-derived cell lines, Lig-8 and Lig-13, are described in detail and compared to properties of adult rat hepatic cell lines derived without Xs supplementation. The Xs-derived cell lines exhibit Xs-dependent asymmetric cell kinetics and Xs-dependent expression of mature hepatic differentiation markers. Interestingly, Lig-8 cells produce progeny with properties consistent with hepatocyte differentiation, while Lig-13 progeny cells have properties consistent with bile duct epithelium differentiation. A stable adult cholangiocyte stem cell line has not been previously described. Consistent with the principles of their derivation, the SACK-derived hepatic cell lines exhibit neither senescence nor tumorigenic properties, and their differentiation properties are stable after longterm culture. These characteristics of SACK-derived stem cell lines underscore asymmetric cell kinetics as an essential adult stem cell property with potential to be the basis for a general approach to expansion and propagation of diverse adult stem cells.

Animals↗

PubMed1996

Protein kinase CK2: biphasic kinetics with peptide substrates.

The regulatory beta-subunit of the protein kinase CK2 modulates the salt optimum for alpha-subunit activity. In the presence of salt the beta-subunit is stimulatory while in the absence of salt it is inhibitory. In the presence of 150 mM NaCl CK2 has linear kinetics (Lineweaver-Burk) for the synthetic substrate RRRDDDSDDD with an apparent Km of 60 microM. In contrast, CK2 displayed biphasic kinetics for the peptide substrate when assayed in the absence of added NaCl. Biphasic kinetics were also obtained for other peptides but not for calsequestrin or casein. Recombinant alpha-subunit had strictly linear kinetics in the absence of added NaCl with an apparent Km of 104 microM. Preincubation of CK2 with ATP/Mg2+ or GTP/Mg2+, but not adenosine/Mg2+ or Mg2+ alone, resulted in kinetics that were near linear. This change in kinetics was dependent on enzyme conditions of low salt CK2 displays biphasic kinetics for peptide substrates, the biphasic kinetics require the presence of the beta-subunit, and ATP/Mg2+ binding reverses the effect.

Adenosine Triphosphate↗

PubMed1987

A comparison of sodium channel kinetics in the squid axon, the frog node and the frog node with BTX using the "silent gate" model.

In this paper it is shown that the very different kinetics measured for the rise of the sodium current which follows a depolarization of the membrane in the squid giant axon, the frog node and the frog node treated with Batrachotoxin may be accurately predicted using only the measured equilibrium and static characteristics for the three preparations and the kinetics measured for the gating charge transfer. The kinetic predictions follow the use of the "silent gate" model for ion channel gating. The model is electrostatic and its chief assumptions are that the channel gate, called here the N-system, has fast kinetics and responds to the gating charge that transfers but not directly to the trans-membrane voltage applied. Because channel gating, corresponding here to the motion of the N-system, does not change its energy in the trans-membrane applied electric field the gating is electrically silent as far as gating charge transfer measurement is concerned. However the probability of gating rises with the quantity of gating charge that transfers due to the electrostatic interaction between the N-system and the gating charge, redistributed under the influence of the applied trans-membrane electric field. With these assumptions the kinetics of sodium channel gating are predictable using only the static and equilibrium characteristics of gating charge and channel activation measured as a function of membrane voltage, and the kinetics of the gating charge transfer. Because of the fast kinetics assumed for the N-system the predicted kinetics are the same for channels with any number of equivalent and independent N-systems or gates acting in parallel.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1995

Are oxygen uptake kinetics at the onset of exercise speeded up by local metabolic status in active muscles?

To assess the rate-limiting factor of oxygen uptake (VO2) kinetics at the onset of exercise, six healthy male sedentary subjects performed repeated one-legged constant-load cycle exercise. The one-legged constant-load exercise test consisted of two 5-min periods of pedalling at an exercise intensity of 50 W, with a 5-min rest between periods (these exercise periods, i.e. first and second exercises, were performed by the same leg). The exercise was then repeated using the other leg. In addition, two-legged incremental exercise was investigated to establish whether VO2 kinetics were affected by slower heart rate kinetics. The incremental exercise test consisted of two-legged pedalling first with the cycle unloaded as a warm-up for 5 min followed by 50-W exercise for 5 min. The exercise intensity was then increased to 100 W for 5 min. During exercise, gas exchange parameters were determined by the breath-by-breath method and cardiac output (Qc) was determined continuously by an impedance technique with a computer-based automated system. To determine the kinetics of heart rate (HR), Qc, and VO2, a best fit procedure was employed using least-squares criteria with a time delay, except during the initial increase. During the one-legged constant-load exercise test, VO2 kinetics were significantly accelerated by repeated exercise using the same leg. On the other hand, when the exercise was changed to the other leg, VO2 kinetics were significantly slower, although Qc kinetics continued to be faster. During the incremental exercise test, although the HR response was slower at the transition from 50-W to 100-W exercise than at the transition from warm-up to 50-W exercise, there were no significant changes in VO2 kinetics.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed2004

Characterisation, asymmetry and reproducibility of on- and off-transient pulmonary oxygen uptake kinetics in endurance-trained runners.

The purpose of this study was three-fold: (1) to characterise both the on- and off-transient oxygen uptake (V(.)O(2)) kinetics in endurance runners during moderate-intensity treadmill running; (2) to determine the degree of symmetry between on- and off-transients; and (3) to determine the reproducibility of V(.)O(2) kinetic parameters in endurance runners. Twelve endurance-trained runners [mean (SD) age 25.2 (4.7) years, body mass 70.1 (9.7) kg, height 179.5 (7.5) cm, ventilatory threshold (V(T)), 3,429 (389) ml.min(-1), maximal V(.)O(2) (V(.)O(2max)) 4,138 (625) ml.min(-1)] performed two multiple square-wave transition protocols on separate days. The protocol consisted of six (three transitions, 15 min rest, three transitions) square-wave transitions from walking at 4 km.h(-1) to running at a speed equivalent to 80% of the V(.)O(2) at the V(T) (80%V(T)). To determine the reproducibility, the protocol was repeated on a separate day (i.e. a test-retest design). Pulmonary gas-exchange was measured breath-by-breath. The V(.)O(2) data were modelled [from 20 s post-onset (or offset) of exercise] using non-linear least squares regression by a mono-exponential model, incorporating a time delay. The on- and off-transient time constants (tau(on) and tau(off)), mean response times (MRT(on) and MRT(off)) and amplitudes (A(on) and A(off)) were obtained from the model fit. On- and off transient kinetics were compared using paired t-tests. The reproducibility of each kinetic parameter was explored using statistical (paired t-tests) and non-statistical techniques [95% limits of agreement (LOA, including measurement error and systematic bias) and coefficient of variation (CV)]. It was found that the tau(on) [12.4 (1.9)] was significantly (P<0.001) shorter than tau(off) [24.5 (2.3) s]. Similarly, MRT(on) [27.1 (1.9) s] was shorter than MRT(off) [33.4 (2.2) s]. With respect to the reproducibility of the parameters, paired t-tests did not reveal significant differences between test 1 and test 2 for any on- or off-transient V(.)O(2) kinetic parameter (P>0.05). The LOA for tau(on) (1.9 s), tau(off) (2.3 s), MRT(on) (1.2 s), MRT(off) (3.2 s), A(on) (204 ml.min(-1)) and A(off) (198 ml.min(-1)) were narrow and acceptable. Furthermore, the measurement error (range, 4.3 to 15.1%) and CV (1.3 to 4.8%) all indicated good reproducibility. There was a tendency for tau(off) to be more reproducible than tau(on). However, MRT(on) was the most reproducible kinetic parameter. Overall, the results suggest that: (1) a multiple square-wave transition protocol can be used to characterise, reproducibly, both on- and off-transient V(.)O(2) kinetic parameters during treadmill running in runners; (2) the phase II time constant is independent of V(.)O(2) (max), and (3) asymmetry exists between on- and off transient V(.)O(2) kinetic parameters.

Adult↗

PubMed2005

On the emergence of biological complexity: life as a kinetic state of matter.

A kinetic model that attempts to further clarify the nature of biological complexification is presented. Its essence: reactions of replicating systems and those of regular chemical systems follow different selection rules leading to different patterns of chemical behavior. For regular chemical systems selection is fundamentally thermodynamic, whereas for replicating chemical systems selection is effectively kinetic. Building on an extension of the kinetic stability, concept it is shown that complex replicators tend to be kinetically more stable than simple ones, leading to an on-going process of kinetically-directed complexification. The high kinetic stability of simple replicating assemblies such as phages, compared to the low kinetic stability of the assembly components, illustrates the complexification principle. The analysis suggests that living systems constitute a kinetic state of matter, as opposed to the traditional thermodynamic states that dominate the inanimate world, and reaffirms our view that life is a particular manifestation of replicative chemistry.

Evolution, Chemical↗

PubMed1977

Electron transport between plastoquinone and chlorophyll Ai in chloroplasts. II. Reaction kinetics and the function of plastocyanin in situ.

The light-induced reaction kinetics of electron carriers between the two light reactions were studied in spinach chloroplasts. 1. The difference spectrum of the absorbance changes of plastocyanin in situ was separated from superimposing absorbance changes by flash titration described in the preceding paper (Haehnel, W. (1973) Biochim. Biophys. Acta 305, 618-631). Relative amounts of 2 : 1 electron equivalents were observed for plastocyanin and chlorophyll a1 (P-700). 2. A balance of the electron equivalents released from reduced plastoquinone and simultaneously accepted by oxidized plastocyanin, cytochrome f and chlorophyll a1 indicated a quantitative electron transfer. Additional electron carriers between plastoquinone and light reaction I can be excluded with an accuracy of about +/-0.3 electron equivalents per light reaction II. 3. The time course of the absorbance changes of plastocyanin was measured at 584 nm with negligible interference with other absorbance changes. The reduction kinetics show an initial lag followed by a rise with a half time of about 20 ms. The redox states of plastocyanin and chlorophyll a1 during this reduction via the rate-limiting step between the light reactions and during oxidation by weak far-red light suggest a true equilibrium constant of about 20. 4. The simultaneous oxidation and reduction kinetics of plastoquinone, cytochrome f, plastocyanin and chlorophyll a1 induced by two successive groups of saturating flashes after far-red illumination were measured. The oxidation kinetics of plastocyanin and the simultaneous reduction kinetics of chlorophyll a1 after the single flashes indicate a quantitative electron transfer with a half time of 200 mus. 5. The fast reduction of chlorophyll a1 by plastocyanin showed no effect of the inhibitors 3-(3',4'-dichlorophenyl)-1,1-dimethylurea and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone or of reduced phenazine methosulfate. But it was completed inhibited after KCN incubation. 6. The oxidation kinetics of cytochrome f were reinvestigated with high time resolution from the difference of absorbance changes at 554 minus 540 nm to minimize the disturbing interference with other absorbance changes. Absorbance changes measured 554 nm alone do not reflect kinetics of cytochrome f. The half time of the oxidation was faster than 40 microns. 7. The observed reaction kinetics gave evidence for a function of cytochrome f between plastoquinone and chlorophyll a1 in parallel to plastocyanin. In addition, they indicate that the greater portion of linear electron transport passes plastocyanin. The complex interaction between cytochrome f and chlorophyll a1 is discussed.

Chlorophyll↗

PubMed1991

Effects of apolipoprotein structure on the kinetics of apolipoprotein transfer between phospholipid vesicles.

The kinetics and mechanism of transfer of 14C-labeled human apolipoproteins A-I, A-II and C-III1 between small unilamellar vesicles (SUV) have been investigated. Ion exchange chromatography was used for rapid separation of negatively charged egg phosphatidylcholine (PC)/dicetyl phosphate donor SUV containing bound 14C-labeled apoprotein from neutral egg PC acceptor SUV present in 10-fold molar excess. The transfer kinetics of these apolipoproteins at 37 degrees C are consistent with the existence of fast, slow and apparently 'nontransferrable' pools of SUV-associated lipoprotein: the transfers from these pools occur on timescales of seconds (or less), minutes/hours and days/weeks, respectively. For donor SUV containing about 15 apoprotein molecules per vesicle and at a donor SUV concentration of 0.15 mg phospholipid/ml incubation mixture, the sizes of the fast kinetic pools for apolipoproteins A-I, A-II and C-III1 associated with donor SUV are 2, 10 and 11%, respectively. The sizes of the slow kinetic pools for these apolipoproteins are 16, 71 and 50%, respectively. The transfer of the various apolipoproteins from the slow kinetic pool follows first order kinetics and the half-time (t1/2) values are in the order: apo C-III1 less than apo A-I. Increasing the number of apoprotein molecules per donor SUV enlarges the size of the fast pool and increases the t1/2 of slow transfer. The differences in the kinetics of apolipoprotein transfer between SUV are consequences of the variations in the primary and secondary structures of the apolipoprotein molecules. The slow transfer of apoprotein molecules is mediated by collisions between donor and acceptor SUV; the rate is dependent on the apoprotein molecular weight with larger molecules transferring more slowly from donor SUV containing the same lipid/protein molar ratio. The hydrophobicity of the apoprotein molecule is also significant with less hydrophobic molecules transferring more rapidly. Further understanding of the differences in the kinetics of transfer of these apolipoproteins will require more knowledge of their secondary and tertiary structures.

Apolipoproteins↗