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Biomedical subjects

M Sinensky

Publications and source records attributed to M Sinensky.

62 records · Page 4Linked to original sources
PubMed1979

Rate limitation of (Na+ + K+)-stimulated adenosinetriphosphatase by membrane acyl chain ordering.

A somatic cell mutant (CR1) of the Chinese hamster ovary cell line (CHO-K1) that is defective in the regulation of cholesterol biosynthesis can be grown under conditions in which plasma membranes from these cells display various cholesterol contents and acyl chain order parameters. The (Na+ + K+)-stimulated adenosinetriphosphatase (ATP phosphohydrolase, EC 3.6.1.3) from these cells was shown to vary in activity by a factor of 10 as the order parameter was varied, and the activity exhibited an exponential dependence on this parameter. Under these conditions the number of Na+,K+-ATPase molecules was shown to remain constant by affinity labeling with [gamma-32P]ATP in the absence of Na+. Control experiments showed that alteration in cholesterol content without change in order parameter did not result in altered enzyme activity. It is concluded that, under our conditions, the rate of catalysis by the Na+,K+-ATPase is determined by the order parameter. These studies suggest a physical mechanism by which variation of membrane lipid composition or other factors that determine membrane lipid acyl chain order parameter can result in variation in membrane enzyme activity.

Acylation↗
PubMed1978

Defective regulation of cholesterol biosynthesis and plasma membrane fluidity in a Chinese hamster ovary cell mutant.

A Chinese hamster ovary cell mutant resistant to killing by 25-hydroxycholesterol is shown to be defective in the regulation of cholesterol synthesis by exogenous cholesterol. When grown with various cholesterol supplements in delipidized serum, the mutant cell exhibits a variation in cholesterol content and plasma membrane fluidity that is not observed in the parental cell type.

Cell Line↗
PubMed1978

Alterations of hepatic Na+,K+-atpase and bile flow by estrogen: effects on liver surface membrane lipid structure and function.

Administration of the synthetic estrogen ethinyl estradiol (17alpha-ethinyl-1,3,5-estratriene-3,17beta-diol) decreases hepatic Na(+),K(+)-ATPase (ATP phosphohydrolase; EC 3.6.1.3) activity and bile flow to 50% and alters the composition and structure of surface membrane lipid in rats. Although the content of phospholipids was not changed by treatment, free cholesterol (130%) and cholesterol esters (400%) were increased in liver surface membrane fractions. These observations correlate with changes in membrane viscosity, as shown by electron spin resonance probes. Both rotational correlation time, using the isotropic probe methyl (12-nitroxyl)stearate, and the order parameter, determined by the anisotropic probe 5-nitroxylstearic acid, were significantly increased in liver surface membrane fractions from rats treated with ethinyl estradiol. Administration of Triton WR-1339, a nonionic detergent that corrects hepatic and serum lipid changes caused by ethinyl estradiol treatment, restored toward normal elevated membrane lipids and viscosity as well as Na(+),K(+)-ATPase activity and bile flow. Although restoration of normal liver surface membrane structure and function may be due to reversal of abnormal lipid composition, detergents also may directly alter membrane enzyme activity. Addition of Triton WR-1339 in vitro increased Na(+),K(+)-ATPase activity and reduced membrane viscosity of surface membranes from rats treated with ethinyl estradiol. Triton had no effect on either parameter in normal membrane preparations. Studies of membrane structure and function both in vivo and in vitro suggest that alterations in lipid composition may alter Na(+),K(+)-ATPase function and bile flow.

Animals↗
PubMed1977

Specific deficit in the synthesis of 6-sulfoquinovsyl diglyceride in Chlorella pyrenoidosa.

It was found that when Chlorella pyrenoidosa was grown on cysteine as the sole sulfur source, it lost the ability to grow photoautotrophically. When grown in the presence of glucose, cysteine-grown cells displayed a doubling time in the light or dark of 45 h, which is identical to that of cells grown on glucose and SO4 in the dark. This suggests that cells grown on cysteine as sole sulfur source can only grow heterotrophically. In support of this hypothesis, it was found that cysteine-grown cells were defective both in vivo and in vitro in CO2 fixation, although O2 evolution in such cells was normal. Assays of the enzymes of the Calvin cycle indicated that the deficit in CO2 fixation could be ascribed to a lowered phosphoribulokinase activity. A total lipid analysis of Chlorella grown on cysteine revealed that such cells showed a 100-fold deficiency in the purportedly chloroplast-associated 6-sulfoquinovsyl diglyceride. This agrees with earlier reports that cysteine could not serve as a precursor of sulfolipid in Chlorella. No other polar lipid was affected. Large amounts of triglyceride, however, were found in cysteine-grown cells. The biosynthesis of triglyceride provides a means of utilizing reduced nicotinamide adenine dinucleotide reducing equivalents not being used for CO2 fixation.

Chlorella↗
PubMed1976

Activation of C55-isoprenoid alcohol phosphokinase from Staphylococcus aureus. II. Biophysical studies.

Physicochemical techniques were used to investigate the activation of C55-isoprenoid alcohol phosphokinase by synthetic lecithins. Complexes of the enzyme with phospholipids were prepared using a method employing sodium dodecyl sulfate as a protein-solubilizing agent. Circular dichorism and the intrinsic fluorescence of the kinase were used as optical probes of protein conformation with these complexes. No evidence for a major lipid-dependent conformational change in the protein was observed when these complexes were studied under conditions where the lipid mesomorphic transitions occurred. EPR studies of mixtures of synthetic lecithins and the C55-isoprenoid alcohol indicated a correlation between kinase activity and the rotational diffusion rate within the hydrophobic phase. It is concluded that the lipid physical state probably does not affect the enzyme activity by altering the protein conformation but more likely does so by affecting the motion of the molecular participants in the reaction.

Binding Sites↗
PubMed1974

Homeoviscous adaptation--a homeostatic process that regulates the viscosity of membrane lipids in Escherichia coli.

E. coli incorporates increasing proportions of saturated and long-chain fatty acids into phospholipids as growth temperature is increased. It was found that this compositional variation results in the biosynthesis of phospholipids that have identical viscosities at the temperature of growth of the cells. This "homeoviscous adaptation" can also be observed in E. coli membrane preparations. Viscosities were determined by use of the electron spin resonance spin-label technique.

Cell Membrane↗
PubMed1971

Temperature control of phospholipid biosynthesis in Escherichia coli.

The higher the growth temperature of Escherichia coli cultures the greater is the proportion of saturated fatty acids in the bacterial phospholipids. When fatty acids are exogenously supplied to E. coli, higher growth temperatures will likewise increase the relative incorporation of saturated fatty acids into phospholipids. One of the steps in the utilization of fatty acids for phospholipid biosynthesis is, therefore, temperature-controlled. The temperature effect observed in vivo with mixtures of (3)H-oleate and (14)C-palmitate is demonstrable in vitro by using mixtures of the coenzyme A derivative of these fatty acids for the acylation of alpha-glycerol phosphate to lysophosphatidic and phosphatidic acids. In E. coli extracts, the relative rates of transacylation of palmityl and oleyl coenzyme A vary as a function of incubation temperature in a manner which mimics the temperature control observed in vivo. The phosphatidic acid synthesized in vitro shows a striking enrichment of oleate at the beta position analogous to the positional specificity observed in phospholipids synthesized in vivo.

Acyltransferases↗