Wide angle x-ray diffraction has revealed that during corpus luteum regression there is a liquid-crystalline to gel phase transition in the phospholipid molecules of the cellular membranes. In the present study we have examined the lipid composition of these membranes and looked for evidence of membrane protein involvement in this change. Lipid analysis of smooth microsomal membranes prepared from rat corpora lutea revealed no significant change in the cholesterol to phospholipid ratio or in the ratio of unsaturated to saturated fatty acids with advancing luteolysis. In addition, there was no clear trend for these changes in the relative proportions of the major fatty acids. Liposomes were prepared from smooth microsomal fractions of regressing rat corpora lutea, and examination of these lipid vesicles by x-ray diffraction revealed that the temperature of the liquid-crystalline to gel phase transition was much lower (approximately 25-30 C) than that for the corresponding microsomes. These observations are consistent with the view that membrane proteins contribute to the ordering of lipid that results in a mixture of liquid-crystalline and gel phases in membranes from regressed corpora lutea.
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