PubMed Health⌕ Search

Biomedical subjects

T J Lewis

Publications and source records attributed to T J Lewis.

At least 19 recordsLinked to original sources
PubMed1998

Pre-steady-state analysis of ATP hydrolysis by Saccharomyces cerevisiae DNA topoisomerase II. 2. Kinetic mechanism for the sequential hydrolysis of two ATP.

In the preceding paper, we showed that DNA topoisomerase II from Saccharomyces cerevisiae binds two ATP and rapidly hydrolyzes at least one of them before encountering a slow step in the reaction mechanism. These data are potentially consistent with two different types of reaction pathways: (1) sequential ATP hydrolysis or (2) simultaneous hydrolysis of both ATP. Here, we present results that are consistent only with topoisomerase II hydrolyzing its two bound ATP sequentially. Additionally, these results indicate that the products of the first hydrolysis are released from the enzyme before the second ATP is hydrolyzed. Release of products from both the first and second hydrolyses contributes to the rate-determining process. The proposed mechanism for ATP hydrolysis by topoisomerase II is complex, having nine rate constants. To calculate values for each of these rate constants, a technique of kinetic parameter estimation was developed. This technique involved using singular perturbation theory in order to estimate rate constants, and consequently identify kinetic steps following the rate-determining step.

Adenosine Diphosphate↗
PubMed1998

Intrathecal sufentanil versus epidural lidocaine with epinephrine and sufentanil for early labor analgesia.

UNLABELLED: Intrathecal sufentanil provides approximately 2 h of excellent labor analgesia with minimal motor blockade. Epidural sufentanil has received less scrutiny but may provide the same benefits as intrathecal sufentanil. In this study, we compared epidural sufentanil 40 microg after a lidocaine with an epinephrine test dose with intrathecal (i.t.) sufentanil 10 microg with respect to onset and duration of analgesia, degree of motor block, side effect profile, and mode of delivery. Seventy ASA physical status I or II parturients in early labor (< or = 4 cm cervical dilation) were randomized to receive either i.t. sufentanil 10 microg with a combined spinal-epidural technique (CSE) or epidural sufentanil 40 microg (e.p.) after epidural catheter placement and testing with 3 mL of 1.5% lidocaine with epinephrine (15 microg). After the administration of analgesia, pain scores and side effects were recorded for each patient at 5, 10, 15, 20, and 30 min, and every 30 min thereafter, by an observer blinded to the technique used. The study period was completed when the patients requested additional analgesia. All patients, except one, achieved adequate analgesia with the initial study dose and satisfactorily completed the study. There were no demographic differences between the two groups. Pain relief was rapid for all patients; pain scores were significantly lower at 5 and 10 min in the i.t. group versus the e.p. group. The mean duration of analgesia was similar between the e.p. group (127 +/- 40 min) and the i.t. group (110 +/- 48 min). No patient experienced any motor block. Side effects were similar between the two groups, except for pruritus-both the incidence and severity were significantly more profound at 5, 10, 15, 20, and 30 min in the i.t. group. There was no difference in time from analgesic to delivery, incidence of operative or assisted delivery, or cervical dilation at the time of redose. For early laboring patients, epidural sufentanil 40 microg after a lidocaine test dose provides analgesia comparable to that of i.t. sufentanil 10 microg with less pruritus. IMPLICATIONS: We compared the efficacy and side effects of intrathecal sufentanil with epidural sufentanil with a local anesthetic test dose for analgesia during labor. Analgesia was equally good, although the intrathecal group experienced more itching.

Adult↗
PubMed1991

Differences in the leucine aminopeptidase activity in extracts from human prostatic carcinoma and benign prostatic hyperplasia.

Extracts of tissue showed that prostatic carcinomas contain less leucine aminopeptidase activity than benign prostatic hyperplasia. This is true when activity is expressed as specific activity (P = 0.0033), specific activity/% epithelium (P = 0.0007), activity/wet weight of tissue (P = 0.0028), or activity/wet weight of tissue/% epithelium (P = 0.0005). Almost all histochemically demonstrable activity is located in the epithelium. Enzymatic activities in extracts and in histochemical preparations showed similar differences between carcinoma and benign prostatic hyperplasia and were related (R = -0.38, P = 0.0400) to Gleason's grades. The transurethrally resected prostate cancers studied contained no well-differentiated tumors and a high proportion of poorly differentiated tumors. Histochemical activity is absent in most prostatic carcinomas and decreased in others. This observation is particularly interesting in view of the growing knowledge of tumor suppressor genes.

Aged↗
PubMed1991

Shear-wave detection of structural effects in aqueous solutions of bovine serum albumin and hemoglobin.

Using cylindrical quartz crystal torsional resonators operating at 39 and 75 kHz to generate shear waves in aqueous solutions of the proteins bovine serum albumin and hemoglobin and the polypeptide poly l-glutamic acid, it has been possible to determine the complex dynamic shear viscosities of the solutions. The effects of concentration, pH, and denaturation using various agents have been studied. It is possible to relate the viscosity and configurational elasticity of the solutions, to the intramolecular and intermolecular forces associated with the of the proteins at frequencies between 60 and 400 kHz and attributed to conformational changes of bovine serum albumin and the quaternary doublet interactions of hemoglobin have been confirmed and emphasized by the use of shear waves.

Animals↗
PubMed1990

Chaotic dynamics in an ionic model of the propagated cardiac action potential.

We simulate the effect of periodic stimulation on a strand of ventricular muscle by numerically integrating the one-dimensional cable equation using the Beeler-Reuter model to represent the transmembrane currents. As stimulation frequency is increased, the rhythms of synchronization [1:1----2:2----2:1----4:2---- irregular----3:1----6:2----irregular----4:1----8:2----...----1:0] are successively encountered. We show that this sequence of rhythms can be accounted for by considering the response of the strand to premature stimulation. This involves deriving a one-dimensional finite-difference equation or "map" from the response to premature stimulation, and then iterating this map to predict the response to periodic stimulation. There is good quantitative agreement between the results of iteration of the map and the results of the numerical integration of the cable equation. Calculation of the Lyapunov exponent of the map yields a positive value, indicating sensitive dependence on initial conditions ("chaos"), at stimulation frequencies where irregular rhythms are seen in the corresponding numerical cable simulations. The chaotic dynamics occurs via a previously undescribed route, following two period-doubling bifurcations. Bistability (the presence of two different synchronization rhythms at a fixed stimulation frequency) is present both in the simulations and the map. Thus, we have been able to directly reduce consideration of the dynamics of a partial differential equation (which is of infinite dimension) to that of a one-dimensional map, incidentally demonstrating that concepts from the field of non-linear dynamics--such as period-doubling bifurcations, bistability, and chaotic dynamics--can account for the phenomena seen in numerical simulations of the cable equation. Finally, we sketch out how the one-dimensional description can be extended, and point out some implications of our work for the generation of malignant ventricular arrhythmias.

Action Potentials↗
PubMed1990

Enzymatic activities in extracts of small (20 +/- 5 mg) samples of prostatic carcinoma.

Previously, we reported that the activities of several enzymes extracted from prostatic cancers were closely related to the histologic grades (Gleason's grades) of the cancers. The survival of patients was more closely correlated with certain enzymatic activities than with Gleason's grades. Unfortunately, because our assays required relatively large amounts of tissue, the increasing proportion of patients who have needle biopsies as their only surgical procedure had to be excluded from our previous study. We now report a method that permits multiple enzyme assays to be carried out with much smaller amounts of tissue. This method will enable us to avoid the selection inherent in the exclusion of patients who have only needle biopsies and should permit the prospective study of most patients with prostatic carcinoma.

Arginase↗
PubMed1989

5'-nucleotidase activity in prostatic carcinoma and benign prostatic hyperplasia.

In light of previous reports of alterations in 5'-nucleotidase activity in neoplastic conditions, 5'-nucleotidase activity was examined histochemically in tissue sections and quantified biochemically in extracts of human hyperplastic prostates and prostatic carcinomas obtained surgically. The 5'-nucleotidase activities per mg protein in extracts of 29 prostatic carcinomas were lower (P less than 0.0005) than in extracts from 10 samples of benign prostatic hyperplasia. The 5'-nucleotidase activity per mg protein in extracts of prostatic carcinoma from the 29 patients correlated (R = -0.369, P = 0.049) with the degree of histological differentiation; the extracts of poorly differentiated carcinomas contained low levels of 5'-nucleotidase. When age and histological differentiation (Gleason's grade) were adjusted, the enzyme activity per mg protein correlated (R = 0.242, P = 0.004) with patient survival. When all three parameters were considered together, i.e., histological grade of the tumor, 5'-nucleotidase extracted from the tumor, and age of the patient, they were found to be mutually complementary for the prediction of patient survival (R = 0.388, P = 0.0001). To our knowledge, this is the first report that prostatic epithelium expresses 5'-nucleotidase; further work will be required to define the reasons for the high levels of activity observed in prostates without cancer and for the decrease in the activity in prostatic carcinoma.

5'-Nucleotidase↗
PubMed1988

The effect of solvation on the ultrasonic absorption of bovine serum albumin solutions.

The role of solvation and structural equilibria in determining the ultrasonic absorption of aqueous solutions of bovine serum albumin has been investigated using chemical agents to denature the protein in solution. The agents used were methanol, ammonium sulphate, and guanidine hydrochloride. Two different techniques of measurement were used to cover the frequency range 200 kHz to 30 MHz. The results show that only guanidine hydrochloride produces a significant change in ultrasonic absorption even though the other agents were used at concentrations close to those which would produce protein precipitation. The conclusion is that in the above frequency range structural and solvation-related equilibria are unimportant and only perturbation of proton-transfer equilibria associated with the protein amino acid residues leads to the bulk of the ultrasonic absorption.

Absorption↗
PubMed1988

Low-frequency ultrasound absorption in aqueous solutions of hemoglobin, myoglobin, and bovine serum albumin: the role of structure and pH.

New measurements on ultrasound absorption of aqueous solutions of the three globular proteins, bovine hemoglobin, horse heart myoglobin, and bovine serum albumin, at 20 degrees C are reported for the frequency range below 1.2 MHz and, in the case of hemoglobin and bovine serum albumin, to a limited extent for the range 2-30 MHz. The effect of protein conformation has been investigated by use of a range of denaturants and by change of pH. A much more detailed description of protein interactions emerges from the low-frequency studies than was possible hitherto. Specific absorption processes for myoglobin and bovine serum albumin peak in the neighborhood of pH 4.2. The process for myoglobin has a relaxation frequency between 1 and 2.6 MHz, while for bovine serum albumin the frequency is as low as 70 kHz. Both processes can be related to conformational changes in the molecules. In the case of hemoglobin, an absorption process with a relaxation frequency of 600 kHz is found that can be attributed to the dissociation equilibrium of the quaternary structure of the molecule and can be considerably enhanced by denaturants or pH change from neutral to acid conditions. Measurements at frequencies down to 200 kHz have also permitted a more thorough investigation of proton-transfer reactions at carboxyl and amino groups in these proteins and of the effect of pH on these reactions.

Absorption↗
PubMed1987

Electrochemical characteristics of platinum electrodes coated with cytochrome b5-phospholipid monolayers.

Platinum electrodes can be coated with cytochrome b5-phospholipid monolayers by the Langmuir-Blodgett technique. Cyclic voltammetry of a series of dyes shows that the coated electrodes become selective for certain electroactive species. The electron transfer reactions of negatively charged species are inhibited at the modified electrode, whereas positively charged species show enhanced reactivity compared with that at a bare metal electrode.

Animals↗
PubMed1986

Ultrasonic absorption of bovine serum albumin solutions in the frequency range 60 to 160 kHz.

The ultrasonic absorption of the globular protein bovine serum albumin in aqueous solution has been measured in the frequency range 60 to 160 kHz using a 2-1 spherical resonator. The effect of pH change and of the denaturants urea, guanidine hydrochloride, and sodium dodecyl sulfate on the absorption loss has been studied. It is concluded that a significant ultrasonic absorption process exists which is related to structural helix-coil transition equilibria. For native protein the maximum loss appears to occur at a frequency at least as low as 70 kHz for a pH of about 4.2. This loss process is distinct from those arising from proton-transfer equilibria perturbations which are manifest at pH 3.2 and 11.6 and at peak frequencies of 400 kHz and above.

Animals↗
PubMed1985

Ultrasonic absorption of bovine serum albumin solutions in the frequency range 200 kHz-1 MHz.

The ultrasonic absorption of solutions of two electrolytes, magnesium and manganous sulfates, and also of the globular protein bovine serum albumin, has been measured using small volume spherical resonators at frequencies in the range 200 kHz-1.2 MHz. The principles of the technique are discussed and absorption values obtained for the electrolyte solutions at 20 degrees C are compared with existing values using other methods. For BSA, measurements were made at 8 degrees C over a range of pH values. Under both acid and alkaline conditions, excess losses at megahertz frequencies were found to persist down to at least 200 kHz. A new loss peak at approximately 400 kHz has been found at pH 3.2.

Absorption↗