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Biomedical subjects

A Tehrani

Publications and source records attributed to A Tehrani.

16 recordsLinked to original sources
PubMed2003

Mechanism of proton transfer inhibition by Cd(2+) binding to bacterial reaction centers: determination of the pK(A) of functionally important histidine residues.

The bacterial photosynthetic reaction center (RC) uses light energy to catalyze the reduction of a bound quinone molecule Q(B) to quinol Q(B)H(2). In RCs from Rhodobacter sphaeroides the protons involved in this process come from the cytoplasm and travel through pathways that involve His-H126 and His-H128 located near the proton entry point. In this study, we measured the pH dependence from 4.5 to 8.5 of the binding of the proton transfer inhibitor Cd(2+), which ligates to these surface His in the RC and inhibits proton-coupled electron transfer. At pH <6, the negative slope of the logarithm of the dissociation constant, K(D), versus pH approaches 2, indicating that, upon binding of Cd(2+), two protons are displaced; i.e., the binding is electrostatically compensated. At pH >7, K(D) becomes essentially independent of pH. A theoretical fit to the data over the entire pH range required two protons with pK(A) values of 6.8 and 6.3 (+/-0.5). To assess the contribution of His-H126 and His-H128 to the observed pH dependence, K(D) was measured in mutant RCs that lack the imidazole group of His-H126 or His-H128 (His --> Ala). In both mutant RCs, K(D) was approximately pH independent, showing that Cd(2+) does not displace protons upon binding in the mutant RCs, in contrast to the native RC in which His-H126 and His-H128 are the predominant contributors to the observed pH dependence of K(D). Thus, Cd(2+) inhibits RC function by binding to functionally important histidines.

Aspartic Acid↗
PubMed2001

Identification of the proton pathway in bacterial reaction centers: decrease of proton transfer rate by mutation of surface histidines at H126 and H128 and chemical rescue by imidazole identifies the initial proton donors.

The pathway for proton transfer to Q(B) was studied in the reaction center (RC) from Rhodobacter sphaeroides. The binding of Zn(2+) or Cd(2+) to the RC surface at His-H126, His-H128, and Asp-H124 inhibits the rate of proton transfer to Q(B), suggesting that the His may be important for proton transfer [Paddock, M. L., Graige, M. S., Feher, G. and Okamura, M. Y. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 6183-6188]. To assess directly the role of the histidines, mutant RCs were constructed in which either one or both His were replaced with Ala. In the single His mutant RCs, no significant effects were observed. In contrast, in the double mutant RC at pH 8.5, the observed rates of proton uptake associated with both the first and the second proton-coupled electron-transfer reactions k(AB)(()(1)()) [Q(A)(-)(*)Q(B)-Glu(-) + H(+) --> Q(A)(-)(*)Q(B)-GluH --> Q(A)Q(B)(-)(*)-GluH] and k(AB)(()(2)()) [Q(A)(-)(*)Q(B)(-)(*) + H(+) --> Q(A)(-)(*)(Q(B)H)(*) --> Q(A)(Q(B)H)(-)], were found to be slowed by factors of approximately 10 and approximately 4, respectively. Evidence that the observed changes in the double mutant RC are due to a reduction in the proton-transfer rate constants are provided by the observations: (i) k(AB)(1) at pH approximately pK(a) of GluH became biphasic, indicating that proton transfer is slower than electron transfer and (ii) k(AB)(2) became independent of the driving force for electron transfer, indicating that proton transfer is the rate-limiting step. These changes were overcome by the addition of exogenous imidazole which acts as a proton donor in place of the imidazole groups of His that were removed in the double mutant RC. Thus, we conclude that His-H126 and His-H128 facilitate proton transfer into the RC, acting as RC-bound proton donors at the entrance of the proton-transfer pathways.

Alanine↗
PubMed2000

Coexpression patterns of mGLuR mRNAs in rat retinal ganglion cells: a single-cell RT-PCR study.

PURPOSE: Eight different subunits of metabotropic glutamate receptors (mGluRs) are known to date. mGluRs have been linked to an extensive list of neuromodulatory effects, depending on which intracellular or membrane-bound effector system is activated. Activation of mGluRs can influence neuronal activity and can result in changes of intracellular Ca2+ homeostasis-that is, changes in factors that are known to be crucial for cellular differentiation and cell death. Because mGluRs are known in modulating both intracellular and intercellular activities, this study was designed to determine which types of mGluRs are coexpressed in a neuron and whether distinct coexpression patterns can be found that reflect the different physiological requirements of a neuron at different stages of development and to learn whether neuronal injury results in adaptive changes of mGluR expression. METHODS: Juvenile and adult rat retinal ganglion cells (RGCs) and adult RGCs after axotomy were analyzed for their gene expression pattern of mGluRs by single-cell reverse transcription-polymerase chain reaction (RT-PCR). RESULTS: Adult RGCs predominantly expressed one or two different mGluR mRNAs, whereas juvenile RGCs coexpressed two and more. mGluR3, -5, and -7 mRNAs were found more frequently in juvenile than in adult RGCs. mGluR6 was detected in juvenile RGCs in low abundance but never in adult RGCs. However, mGluR6 was expressed in adult RGCs after axotomy. mGluR1 and -7 were also found more frequently in axotomized RGCs than in the adult control group. CONCLUSIONS: All types of mGluR mRNAs are expressed in RGCs. This is in contrast with previous in situ hybridization and immunohistochemical studies in which expression of mGluR3, -5, and -6 was not reported. The expression of some mGluR mRNAs seems to be developmental, although no distinct copatterns were found.

Aging↗
PubMed1998

Protein organization on the PHA inclusion cytoplasmic boundary.

Polyhydroxyalkanoate (PHA) cellular inclusions consist of polyesters, phospholipids, and proteins. Both the polymerase and the depolymerase enzymes are active components of the structure. Recently, proteins associated with these inclusions have been described in a number of bacterial species. In order to further clarify the structure and function of these proteins in relation to polymer inclusions, ultrastructural studies of isolated polymer inclusions were initiated. The surface boundary characteristics of polymer inclusions, produced by several genera of bacteria, two different Pseudomonas putida deletion mutants and by Escherichia coli recombinants, were examined. The recombinant E. coli carried either the PHB biosynthesis operon (phaCAB) from Ralstonia eutropha alone, or both this operon and a gene encoding an inclusion surface protein of R. eutropha (phaP). The results support two suggestions: (i) specific genes in the PHA gene cluster code for the proteins forming the surface boundary arrays which characterize the polymer inclusion; and (ii) transfer of such a gene would result in subcellular compartmentalization of accumulating polymer. Although the proteins appear to serve a similar function among different genera, nevertheless, the different surface proteins are encoded by a variety of non-homologous genetic sequences.

Acids, Acyclic↗
PubMed1996

Intracellular depolymerase and polyhydroxyoctanoate granule integrity in Pseudomonas oleovorans.

When polyhydroxyoctanoate (PHO) was produced by Pseudomonas oleovorans during a regimen of intermittent feeding on octanoic acid, there was a significant change in both the polymer associated proteins and the composition of the enclosed polymer. The polymer granules were isolated with their protein coat intact and the enzymatic hydrolysis of the polymer within this cell free system was determined. The degradation rate for the PHO in these native granules reached a maximum of 1.17 mg/h at an optimum pH of 9 when incubated at 30 degrees C. A study of the effect of various inhibitors on depolymerase activity suggested that the enzyme most likely has disulfide linkages and serine residues at its active site. Ultrastructure studies suggested this loss of enzyme activity was correlated with significant organizational degeneration in the proteins associated with the PHO inclusion body. Once solubilized from the granule, the depolymerase itself remained enzymatically active, and addition of this released material to other granule preparations increased the rate of polymer granule degradation. Similarly, when colloidal suspensions of purified, amorphous PHO were placed in contact with that depolymerase, they also underwent rapid degradation. In contrast, when crystalline solvent-cast PHO films were placed in contact with this enzyme, no degradative activity was observed.

Bacterial Proteins↗
PubMed1988

Demineralization potential of different concentrations of gelatinized wheat starch.

Six subjects wore intraoral devices carrying bovine enamel blocks covered with a layer of Streptococcus mutans. They swished solutions of 5% glucose or maltose, or sols or gels of 3, 5, 10, 15, or 20% gelatinized wheat starch in the mouth for 3 X 1 min. Demineralization was measured after 45 min by determining the change in iodide permeability (delta Ip) of the enamel. Spittings of the administered materials and samples of saliva, taken at intervals during the test, were analyzed for maltose, and the time of clearance was calculated. Demineralization was greatest for glucose followed by maltose, the starch gels, and the sols. The latter gave close to zero scores. The salivary clearance time increased with increase in concentration of the starch. Although the starch was hydrolyzed rapidly in the mouth, its oral retentiveness was greater than that of the maltose rinse. Demineralization was closely correlated with the final pH of the S. mutans cell layer. The data indicate that the starch in baked or cooked foods may have a significant demineralization potential and that it enhances oral retentiveness.

Animals↗
PubMed1986

Enamel demineralization and the length of intra-oral exposure to different concentrations of glucose or sucrose.

Six subjects wore intra-oral devices carrying eight bovine enamel blocks which were covered with a layer of S. mutans. They rinsed their mouths for five sec, one min, or multiples of one min with a 5 or 10% glucose (G) solution. Demineralization was measured after 45 min by determining the change in iodide permeability (delta Ip) of the enamel. In addition, saliva samples, taken at intervals during the test, were analyzed for G, and the time of clearance (tc) was calculated. Demineralization scores (delta Ip) were consistently greater from the 10% than from the 5% G solution. Both solutions produced an increase in delta Ip with an increase in the rinsing time (tr). The G remaining in saliva after the rinse did not significantly affect delta Ip. The delta Ip scores showed good correlation with the final pH of the S. mutants cell mass, r = -0.77. For each rinse solution, the scores also showed good correlations with tr (r = 0.87 and 0.79) and much weaker correlations with tr + tc (r = 0.44 and 0.53). Continuous in vitro exposure to 1, 5, or 10% solutions of sucrose (S) in saliva for 30 min or more caused a linear increase in delta Ip with time with no concentration effects. A linear increase was also observed in vivo when a one-minute mouth-rinse with 10% solution of S was administered every 30 min. The findings indicate that significant demineralization may occur while carbohydrate foods are consumed, and that brushing the teeth or rinsing the mouth after meals may not be as effective against caries as is generally believed.

Adult↗
PubMed1985

Effect of some salts of calcium, sodium, potassium, and strontium on intra-oral enamel demineralization.

A newly-developed intra-oral enamel demineralization test was used to evaluate the effect of supplementation of a 10% sucrose solution with various components on enamel demineralization induced by the sucrose. Five human subjects wore a palatal prosthesis holding eight blocks of subsurface bovine enamel covered with a layer of S. mutans cells. The test involved rinsing with sucrose solution or with sucrose solution supplemented with 0.162 mol/l of different calcium salts or equivalent concentrations of Na-, K-, and Sr salts; rinsing was for one min at times zero and 45 min of the 90-minute test period. Ca-propionate, Ca-acetate, and Ca-levulinate completely inhibited sucrose-induced enamel demineralization; Ca-chloride, Ca-lactate, and Ca-ascorbate gave from 65-75%, and K-acetate, Na-lactate, and Sr-lactate 39, 25, and 18% inhibition, respectively. Consideration of the anion dissociation constants and the Ca-anion association constants of the salts suggests that the observed inhibition is caused mainly by common ion effects and, to a lesser extent, by buffer effects.

Adult↗
PubMed1985

Sugar clearance from saliva and intra-oral spaces.

The clearance of glucose from saliva was compared with that from spaces located in an intra-oral appliance. The spaces (N = 8) were similar in shape and size to posterior interdental spaces and could be sampled with a microsyringe without removing the appliance from the mouth. The subjects (N = 5) rinsed their mouths with solutions of 5, 10, or 25% glucose. Samples were obtained from a pair of spaces and from saliva at standard intervals and were analyzed for glucose. The rate of glucose clearance was consistently slower from the spaces than from bulk saliva, as shown by higher concentrations and longer clearance times for the spaces. The clearance curves were similar in shape and were rectilinear when the logarithms of the concentrations were plotted against time. The rate of clearance from the spaces appears to be characteristic for an individual, as was also true of salivary clearance.

Adult↗
PubMed1983

Enamel demineralization by mouthrinses containing different concentrations of sucrose.

Intra-oral enamel demineralization tests, involving rinsing with sucrose solutions (5, 1, 0.5, or 0.3%) at varying frequencies during a 1.5-hour period, showed that demineralization was a function of the total sugar dose (frequency of rinsing times concentration). Two rinses with a 5% solution caused demineralization, nine rinses were needed in the case of a 0.5% solution, and none occurred after nine rinses with a 0.3% solution. Remineralization was also observed and apparently occurred after exhaustion of the sucrose supply.

Adult↗
PubMed1983

Enamel demineralization potential of dietary carbohydrates.

Mouthrinses with 5% solutions of different sugars were tested for their demineralizing effect on blocks of bovine enamel covered with a layer of S. mutans cells and carried in a palatal prosthesis. The extent of demineralization, as measured by the permeability of the enamel to iodide, was similar for fructose, glucose, and sucrose, less for maltose and lactose, and least for galactose.

Adult↗
PubMed1982

Intraoral mineralization of abraded dental enamel.

The iodide permeability (Ip) of abraded bovine enamel increased after short exposure to an acid buffer and decreased after short exposure to a mineralizing solution. Intraoral exposure gave a marked decrease in Ip after one h and a continued lesser decrease after two and three h. In vitro exposure to fresh and dialyzed saliva and various undersaturated solutions indicated that the intraoral decrease was due to mineralization rather than to pellicle formation. Analysis of the data also indicated that part of the mineral formed intraorally was more loosely bound to the enamel than that formed from an inorganic mineralizing solution. The rapid rate of the initial phase of intraoral mineralization shown in this study reveals a powerful mechanism for protecting the dentition against demineralization.

Acetates↗
PubMed1982

The relationship among the permeability to iodide, pore volume, and intraoral mineralization of abraded enamel.

The results indicate that Ip measurements are fairly closely related to the pore volume of the enamel to a depth of about 13 micrometer. A previous finding - that intraoral exposure produced substantial mineralization of abraded enamel after one h, and a slower rate of mineralization during the next two h - was confirmed, and it was shown that the rate of mineralization decreased with a decrease in the pore volume of the enamel. The rapid rate of intraoral mineralization represents a powerful mechanism for maintaining a fully mineralized enamel surface. The sensitivity of the Ip method demonstrated in this study, and the finding that Ip measurements relate to the pore volume of the enamel, coupled with previous findings that the increase in Ip produced by mild acid etching of intact enamel is proportional with the amount of dissolved Ca, indicate that the method provides valid measurement of intraoral de- and remineralization.

Acid Etching, Dental↗