Structure-stabilizing effect of albumin on rat ovarian LH/hCG receptors

The stabilizing effect of albumin on structure-functional alteration of LH/ hCG receptors was analyzed by thermal perturbation technique. On exposing t he membranes to bovine serum albumin (BSA) the heat inactivation profile of hCG-binding sites was shifted to a temperature higher by about 5 degrees C (T-50 values). The receptor destabilizing action of arachidonic and oleic acids incorporated into ovarian membranes and reversal of this effect when BSA was used as fatty acid scavenger, may indicate that free fatty acids ar e responsible for the thermal instability of hCG-binding sites. This presum ption was corroborated by digestion of membranes with phospholipase A(2) (P LA(2)). This enzyme exerted effects on the thermal stability of the recepto r protein resembling those observed upon insertion of fatty acids. The memb rane fluidization induced by arachidonic acid can be reversed by BSA. Howev er, alterations of lipid fluidity in membranes were not found to be a neces sary prerequisite for stabilization of the LH/hCG receptor structure. Fluor escence quenching studies indicated that incorporation of oleic acid or dig estion of membrane phospholipids with PLA(2) elevated the accessibility of fluorophores for acrylamide. BSA scavenging of free fatty acids approached the quenching rate of control membranes. Analysis of fluorescence of membra nes bound to monodansylcadaverine probe revealed that the negative surface charge derived from free fatty acids resulted in destabilization of the rec eptor protein. The effects of free fatty acids on membranes suggest that al tered lipid-protein interactions may directly affect the stability of the L H/hCG receptor structure. (C) 1999 Elsevier Science B.V. All rights reserve d.