Binding of chemotactic peptide to the outer surface and to whole human spermatozoa with different affinity states

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Abstract
Binding of N‐formyl‐methionyl‐L‐leucyl‐[3H]phenylalanine (fML[3H]Ph) to human ejaculated spermatozoa and to its isolated plasma membrane was studied. Our data confirm the presence of specific receptors for f‐MLPh in the human spermatozoa and suggest that whole spermatozoa receptors exist in two affinity states, one high‐affinity, low‐capacity specific receptor (Kd = 12.3 ± 0.5 nM, n = 22,285 ± 65,008 binding sites per sperm cell) and a second one (Kd = 700 ± 47 nM) that is not saturable, indicating a low‐affinity, high‐capacity nonspecific site. In contrast, sperm membrane showed only one class of binding site (Kd = 6.4 ± 0.12 nM), which was statistically different from that of the high‐affinity binding site of intact spermatozoa. To explain this difference we discuss the possibility that first, the two binding affinities represent two interconvertible states of a single receptor population, which, depending on the metabolic activity of spermatozoa, may change its physicochemical properties; or second, they reflect two different processes, binding and/or transport into the spermatozoa.