Binding of 17-β-estradiol to the outer surface and nucleus of human spermatozoa

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Abstract
The binding of 3H-17-β-estradiol to human ejaculated spermatozoa and to its subcellular structures was studied. The binding kinetics of the labeled steroid to whole spermatozoa followed a parabolic pattern. Scatchard-type plots showed the presence of high-affinity binding sites (1.56 ± 0.23 × 104 per sperm cell) with an apparent Kd of 6.6 × 10−10 M. In competition experiments testosterone was partially effective in decreasing 17-β-estradiol binding, whereas progesterone and 17-α-estradiol were ineffective. Study of membrane fractions obtained from estradiol-labeled spermatozoa showed that under saturating conditions 75–84% of the bound steroid was bound to sperm membranes. Nuclear fractions obtained from estradiol-labeled spermatozoa showed only 10% of the total bound radioactivity. When isolated sperm nuclei were incubated in the presence of the purified receptor-17-β-estradiol complex obtained from the high-speed supernatant of human uterus almost no transfer of radioactivity to the nuclei was observed.