Developmental Profile of a Caltrin-Like Protease Inhibitor, P12, in Mouse Seminal Vesicle and Characterization of Its Binding Sites on Sperm Surface1

Abstract

We examined the developmental profile of a kazal-type trypsin inhibitor (P12) of M_r 6126 in mouse seminal vesicle, characterized its binding sites on the surface of sperm, and assessed its effect on Ca²⁺ uptake by spermatozoa. Among the genital tracts of adult mice, P12 was found only in the male accessory glands including seminal vesicle, coagulating gland, and prostate. It was immunolocalized on the luminal epithelium of the primary and secondary folds in both the seminal vesicle and coagulating gland, and on the folds projecting into the lumen of the glandular alveolus in the prostate. The protein and its RNA message in seminal vesicle did not appear in the prepubertal period, but expression coincided with maturation. Castration of adult mice resulted in cessation of P12 expression. Treatment of the castrated mice with testosterone propionate in corn oil restored the protein expression in the seminal vesicle. Spermatozoa collected from caudal epididymis were devoid of P12. Cytochemical study illustrated a P12-binding region on the anterior acrosomes of cells preincubated with P12. Analysis of equilibrium data from the binding assay using ¹²⁵I-P12 with a Scatchard plot showed a single type of P12-binding sites on sperm, with an apparent dissociation constant of 70.15 ± 5.25 nM and the capacity of 1.49 ± 0.06 × 10⁶ binding sites/cell. The protein could serve as a calcium transport inhibitor to suppress a great extent of Ca²⁺ uptake by spermatozoa. The immunohistochemical staining patterns of testis revealed that the P12-binding sites appeared on postmeiotic cells such as spermatids and spermatozoa, but were absent in Leydig cells, Sertoli cells, spermatogonia, and spermatocytes in seminiferous tubules.