Effects of the polyamine spermine on binding of follicle-stimulating hormone to membrane-bound immature bovine testis receptors

Abstract

The effect of the polyamine, spermine, on the interaction of human 125I-labeled FSH with membrane-bound receptors derived from bovine calf testes has been examined. Concentrations of spermine less than 0.01 M resulted in a slight but insignificant ( P > 0.10) enhancement of FSH binding to membrane receptors, while concentrations of 0.01 M and above caused a progressive reduction of FSH binding. Membrane receptors incubated in the presence of spermine at concentrations inhibitory to human 125I-FSH binding (0.01–0.04 M) resulted in an 8–50% decrease in the apparent FSH receptor concentration and a 10–65% decrease in the affinity constant as determined by computerized analysis of the isothermic ligand-binding data. Within the temperature range 4–20°C, simultaneous addition of spermine (0.025 M) increased the reversibility of human 125I-FSH binding approx. 10% ( P < 0.005). Delayed addition of spermine (0.01–0.04 M) resulted in a dose-related dissociation of human 125I-FSH already bound to its receptor ( P < 0.05). However, preincubation of membrane receptors with spermine (0.002–0.04 M) at 4°C or 34°C followed by washing and addition of human 125I-FSH, resulted in an increase in hormone binding ( P < 0.05) over that of controls. If membrane receptor was incubated at 34°C with spermine in the absence of radioligand, the usual loss of hormone binding was reduced ( P < 0.05), while membrane receptor incubated with spermine at 4°C exhibited hormone binding greater ( P < 0.05) than that observed before treatment. Thus, the mechanism of inhibition of human 125I-FSH binding to membrane receptors appears to be correlated with an increase in reversibility of the membrane receptor-human 125I-FSH complex and is expressed as a decrease in the calculated receptor concentration and affinity constant of that interaction. Second, spermine appears to stabilize the membrane receptor in the absence of ligand, presumably through a membrane effect. These data suggest that spermine, and possibly other polyamines, which are endogenous to eukaryotic cells and undergo increases in concentration following stimulation by trophic hormone may play a role in the modulation of the ligand-membrane receptor interaction, in part, through direct effects on the membrane and/or the receptor.