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Biomedical subjects

B J Litman

Publications and source records attributed to B J Litman.

At least 55 records · Page 3Linked to original sources
PubMed1981

Rhodopsin-phospholipid interactions: dependence of rate of the meta I to meta II transition on the level of associated disk phospholipid.

Solubilization of retinal rod outer segment disk membranes in octyl glucoside was employed to prepare rhodopsin samples with varying amounts of associated disk phospholipid. Flash photolysis studies were carried out on these samples to determine the dependence of the meta I to meta II transition kinetics on the level of associated phospholipid. The rate constant for the formation of meta II increased from 6.9 X 10(3) to 19.5 X 10(3) s-1 as the molar ratio of phospholipid per rhodopsin fell from 35 to 5. The activation free energy for this process had a linear dependence on the level of phospholipid, with a slope of 24 cal/mol of rhodopsin-associated phospholipid. A variety of evidence suggests that rhodopsin undergoes a reversible conformation change during the meta I to meta II transition. No evidence was found for an enhanced effect on the activation free energy for this conformation change at the level of associated phospholipid which corresponds to the formation of a phospholipid boundary layer around rhodopsin.

Animals↗
PubMed1977

A simple method for the preparation of homogeneous phospholipid vesicles.

A new method is described for the preparation of homogeneously sized, single-lamellar phospholipid vesicles. This method, wich is based on differential high-speed ultracentrifugation, has the advantages of a higher vesicle yield without dilution and rapidity of preparation when compared to the molecular-sieve technique. The homogeneity of vesicle dispersions, prepared by this new method, is examined by several physical techniques and found to be comparable to the best samples prepared by molecular-sieve chromatography.

Diffusion↗
PubMed1977

A large-scale purification of phosphatidylethanolamine, lysophosphatidylethanolamine, and phosphatidylethanolamine, and phosphatidylcholine by high performance liquid chromatography: a partial resolution of molecular species.

Egg yolk phospholipids, on a 10 g scale, were resolved by high-performance liquid chromatography on an 8-m silica column with elution by a stepwise chloroformmethanol gradient into homogenous phosphatidylcholine, phosphatidylinositol, phosphatidylethanolamine, and lysophosphatidylethanolamine fractions. Within these fractions, partial resolution on the basis of fatty acyl side chain composition was achieved.

Chromatography, High Pressure Liquid↗
PubMed1976

Microviscosity of the hydrocarbon region of the bovine retinal rod outer segment disk membrane determined by fluorescent probe measurements.

The microviscosity of the hydrocarbon region of the bovine retinal rod outer segment disk membrane was determined by measuring the anisotropy of fluorescence from the probe 1,6-diphenyl-1,3,5-hexatriene. The microviscosity ranged from 1.4 P at 40 degreeC to 15 P at O degree C, and no phase transition was observed in this temperature range. Bleaching of rhodopsin in the disk membrane produced no change in the microviscosity within the limits of error of our measurement (not equal to 5%). The presence of retinal in the disk membrane caused string quenching of the probe fluorescence. Removal of retinal from bleached disks produced a threefold increase in total fluorescence intensity and only a 10% drop in fluorescnece anisotropy, suggesting that large lateral gradients in microviscosity do not exist in the disk membrane. Bilayers prepared from extracted disk lipids had a microviscosity which was about one-fourth that of the intact disk membrane, demonstrating that rhodopsin hinders the mobility of the hydrocarbon chains of the disk phospholipids or egg phosphatidylcholine had identical microviscosities despite the much higher degree of unsaturation of the disk phospholipids.

Animals↗
PubMed1976

Alkyl glucosides as effective solubilizing agents for bovine rhodopsin. A comparison with several commonly used detergents.

The suitability of octyl and decyl-beta-D-glucoside as solubilizing agents for the bovine retinal rod outer segment disc membrane was investigated and compared to that of hexadecyltrimethylammonium bromide, N,N-dimethyldodecylamine oxide, Emulphogene BC-720 and digitonin. The properties measured included the thermal stability of rhodopsin, regenerability of bleached rhodopsin by addition of 11-cis-retinal, and the rate of denaturation of bleached rhodopsin as measured by changes in the ultraviolet CD spectrum. Denaturing tendencies of the detergents were also evaluated by observing their effects on the absorption and CD spectra of sperm whale metmyoglobin. Our results demonstrate that octyl glucoside is superior to the other detergents, with the possible exception of digitonin, by the above criteria. Unlike digitonin, however, octyl glucoside affords rapid solubilization of the disc membrane and is itself highly soluble. Decyl glucoside has properties equivalent or superior to octyl glucoside, but salts and buffers interfere with its ability to solubilize the disc membrane. The well defined chemical composition, ease of removal by dialysis, and non-denaturing properties of the alkyl glucosides make them attractive detergents for membrane research.

Animals↗
PubMed1975

Surface distribution of the fatty acid side chains of phosphatidylethanolamine in mixed phospholipid vesicles.

A method has been developed for the selective determination of the fatty acid side chain distribution associated with the amino containing phospholipids located in the inner and outer surfaces of membranes. Using sonicated phosphatidylethanolamine/phosphatidylcholine vesicles as a model, the analysis consists of selective labeling of the outer surface amino groups with the membrane impermeable reagent 2,4,6-trinitrobenzenesulfonic acid. Outer and inner surface phosphatidylethanolamine fractions are separated by thin-layer chromatography. Analysis of methyl esters derived from these two fractions, by gas-liquid chromatography, yields the fatty acid side chain distribution. Our results show that there is no mol fraction dependence of the incorporation of any specific fatty acid side chains of egg yolk phosphatidylethanolamine into the vesicle or any preferential distribution of these side chains in the inner or outer vesicle surface. The surface distribution of the egg yolk phosphatidylethanolamine molecules in these vesicles appears to be determined by the head group packing requirements and not the fatty acid side chain composition.

Chromatography, Gas↗
PubMed1975

Transbilayer exchange of phosphatidylethanolamine for phosphatidylcholine and N-acetimidoylphosphatidylethanolamine in single-walled bilayer vesicles.

A preparation of small single-walled liposome vesicles containing a 9:1 mole ratio of phosphatidylcholine to phosphatidylethanolamine was subjected to reaction with the membrane-impermeable reagent, isethionyl acetimidate hydrochloride. This reagent converted 90% of the external phosphatidylethanolamine groups to the amidine derivative, leaving the mole ratio of unreacted phosphatidylethanolamine to phosphatidylcholine on the outside surface of the vesicle much lower than that on the inside surface. Equilibration of phosphatidylethanolamine across the bilayer was then measured as a function of time by monitoring the appearance of phosphatidylethanolamine on the outside surface utilizing the reaction of the amino groups with 2, 4, 6-trinitrobenzenesulfonic acid. The results show that no new phosphatidylethanolamine appeared on the external surface of the vesicles over a period of 12 days at 22 degrees. A conservative estimate of the precision of the measurements is +/- 10%. On this basis, the estimated half-time for the equilibration of phosphatidylethanolamine across the bilayer of these vesicles must be at least 80 days at 22 degrees.

Binding Sites↗
PubMed1975

The optical activity of D-erythro-sphingomyelin and its contribution to the circular dichroism of sphingomyelin-containing systems.

Circular dichroism studies on bovine brain sphingomyelin show the presence of a strong negative cotton effect below 200 nm, the position and magnitude of which depend on the physical state of the lipid. This cotton effect is thought to arise from the pi-pi transition of the amide group in the sphingomyelin backbone. The sphingomyelin contribution to the observed ellipticity of membranes and lipoprotein complexes depends on the mol fraction of amide groups present as sphingomyelin: this contribution is calculated to be less than 2% in the case of serum high density lipoprotein and the order of 20% below 200 nm in the case of the erythrocyte ghost membrane. Due to the similarity of the CD spectrum of sphingomyelin to that of a random coil polypeptide, use of uncorrected ellipticity data is expected to lead to an overestimate of the random coil content of proteins in systems containing a high sphingomyelin content.

Animals↗