Trypsin/acrosin inhibitor activity of rat and guinea pig caltrin proteins.Structural and functional studies

Dramatic inhibition of trypsin activity by rat caltrin and guinea pig caltr in I was spectrophotometrically demonstrated using the artificial substrate benzoylarginyl ethyl ester. Approximately 6% and 21% of residual proteolyt ic activity was recorded after preincubating the enzyme with 0.22 and 0.27 PM rat caltrin and guinea pig caltrin I, respectively. Reduction and carbox ymethylation of the cysteine residues abolished the inhibitor activity of b oth caltrin proteins. Rat caltrin and guinea pig caltrin I show structural homology with secretory trypsin/acrosin inhibitor proteins isolated from bo ar and human seminal plasma and mouse seminal vesicle secretion and share a fragment of 13 amino acids of almost identical sequence (DPVCGTDCH/K/ITYG/ AN), which is also present in the structure of Kazal-type trypsin inhibitor proteins from different mammalian tissues. Bovine, mouse, and guinea pig c altrin II, three caltrin proteins that have no structural homology with rat caltrin or guinea pig caltrin I, lack trypsin inhibitor activity. Rat calt rin, guinea pig caltrin I, and the mouse seminal vesicle trypsin inhibitor protein P12, which also inhibits Ca2+ uptake into epididymal spermatozoa (m ouse caltrin I), bound specifically to the sperm head, on the acrosomal reg ion, as detected by indirect immunofluorescence. They also inhibited the ac rosin activity in the gelatin film assay. Caltrin I may play an important r ole in the control of sperm functions such as Ca2+ influx in the acrosome r eaction and activation of acrosin and other serine-proteases at the proper site and proper time to ensure successful fertilization.