Effect of phospholipase C induced hydrolysis of phospholipids on membrane-bound and water-soluble LH/hCG receptors in porcine corpora lutea

Effect of phospholipids on the reconstitution and thermal stability of delipidated rat ovarian luteinizing hormone/human chorionic gonadotropin receptors in proteoliposomes

The role of lipids and a possible structure-functional alteration of delipidated LH/hCG receptor reconstituted into proteoliposomes was analyzed by thermal perturbation techniques. Delipidated receptor lost to a great extent its binding activity and thermal stability. The LH/hCG receptor was almost fully reactivated by the reconstitution into proteoliposomes with phosphatidylcholine (PC) and sphingomyelin (SpM) and partly with phosphatidylethanolamine (PE) and phosphatidic acid (PA). The heat inactivation profile of delipidated LH/hCG binding sites was shifted to a lower temperature by about 4 degrees C (T50 values). Thermal inactivation of the receptor by delipidation was entirely inverted by treatment with soybean PC, dioleoyl PC and dipalmitoyl PC and partially with SpM. The presence of negatively charged phospholipids, phosphatidylserine (PS), phosphatidylglycerol (PGl) and PA, did not change the heat-inactivation profile of the LH/hCG receptor modified the differential scanning calorimetric profile and the quenching of protein fluorescence characteristic for control proteoliposomes. Delipidation increased membrane lipid rigidity. Reconstitution of delipidated proteoliposomes with soybean PC, dioleoyl PC, PGl, PS and PA decreased, and that of dipalmitoyl PC, lysoPC, SpM and cholesterol increased the degree of fluorescence polarization of DPH of proteoliposomes. The different action of phospholipids on the reconstitution, thermal inactivation of the receptor and membrane lipid fluidity in proteoliposomes suggests that lipid fluidity is not related to the stabilizing action of phospholipids on the LH/hCG receptor.

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

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 degrees (T50 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, dioleoylphosphatidylcholine (DOPC) and dipalmitoylphosphatidylcholine (DPPC), partly with phosphatidylethanolamine (PE) and sphingomyelin (SpM), but not with phosphatidylserine (PS), phosphatidylglycerol (PGl) 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 PGl 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.