Properties of the Luteinizing Hormone Receptor of Isolated Bovine Corpus Luteum Plasma Membranes

Abstract

Abstract Luteinizing hormone binding sites with high affinity and specificity for ovine-luteinizing hormone have been shown to be present in a purified plasma membrane preparation obtained from bovine corpus luteum. The specific binding of 125I-luteinizing hormone to the membranes is a saturable process with respect to 125I-luteinizing hormone. Native luteinizing hormone competes for the binding in a way expected from the biological identity of the 2 molecules. Human chorionic gonadotropin and pregnant mare serum gonadotropin, two gonadotropins which have luteinizing hormone activity, compete for the binding site of luteinizing hormone, with an affinity which is less than that of native luteinizing hormone. The same holds for the α and β chains of luteinizing hormone which showed, respectively, 100 and 200 times less affinity than luteinizing hormone for its binding site. Follicle-stimulating hormone, which does not have intrinsic luteinizing hormone activity, does not compete for the binding site of luteinizing hormone to an extent greater than its contamination by luteinizing hormone allows. The binding of 125I-luteinizing hormone is temperature-dependent and reaches its maximum in 10 min at 37°. The rate constant of the luteinizing hormone-membrane association (2.17 x 106 m-1 s-1) and dissociation (2.46 x 10-3 s-1) have been measured independently at 23 and 10°. The dissociation constant (1.13 x 10-9 m) based on these rate constants is similar to that (3 x 10-9 m) calculated separately from equilibrium data. Measurement of the rate constants at various temperatures gives similar values for the dissociation constant. This shows that the decrease in dissociation rate is proportionately the same as the decrease in association rate. Binding is maximal at pH 7.6 and is not affected by Ca2+ concentration in the range of 0.1 to 20 mm. The effects of different enzymatic preparations on the binding site of luteinizing hormone have been investigated. It is not affected by DNase, trypsin, chymotrypsin, pepsin, and collagenase. Treatment of the membrane preparations by neuraminidase increased the binding capacity for luteinizing hormone by 2-fold. Phospholipase C, as well as phospholipase A, decreases it by half.