Effects of partial delipidation of rat ovarian membranes on thermal stability of LH/hCG receptors

Abstract

The role of lipids and of possible structure-functional properties of the LH/hCG receptor were analyzed by thermal perturbation techniques in its native membrane environment. A method for the reversible removal of lipids from membranes with a mild detergent Tween 20 was developed. The receptor was reactivated with phosphatidylcholine (PC) by its reconstitution into proteoliposomes. The heat inactivation profile of LH/hCG binding sites in delipidated membranes was shifted to a temperature lower by approx. 8 C° ( T 50 values). Thermal inactivation of the delipidated LH/hCG receptor was found to be a quick process. Occupation of receptor binding sites by the agonist before thermal perturbation induced stabilization of the receptor. Thermal inactivation of the receptor by delipidation was fully reversed by treatment with soybean PC, diolcoylphosphatidylcholine (DOPC) and dipalmitoylphosphatidylcholine (DPPC), partly with phosphatidylethanolamine (PE) and sphingomyelin (SpM), but not with phosphatidylserine (PS), phosphatidylglycerol (PGI) or cholesterol. Delipidation modified the differential scanning calorimetric profile characteristic of control membranes. Delipidation of ovarian membranes also increased membrane lipid rigidity. The addition of PC, DOPC and PS to delipidated membranes decreased, that of DPPC and SpM increased, while PGI did not change the degree of fluorescence polarization of DPH, suggesting that membrane lipid fluidity was not involved in the stabilizing action of specific phospholipids against thermal inactivation of the ovarian LH/hCG receptor.