SERINE-PROTEASE INHIBITORS (SERPINS) IN HUMAN SEMINAL PLASMA - CONCENTRATIONS AND INHIBITION OF ACROSIN

Acrosin is a trypsin-like serine protease present in its zymogen form in the acrosome of spermatozoa. The activated enzyme is thought to dig est a pathway for the sperm through the zona pellucida of the ovum dur ing the process of fertilisation. We have shown that in a purified sys tem boar acrosin was inhibited by human protein C inhibitor with an ap parent second order rate constant (k(app)) of 3.7x10(4)M(-1)s(-1). Pro tein C inhibitor is present in high concentrations in seminal plasma a nd endogenous protein C inhibitor was found in the immediate vicinity of disrupted acrosomal membranes of washed human spermatozoa. This ser pin could therefore function as a scavenger of prematurely activated a crosin in the male reproductive tract. Since little is known about the interaction of acrosin with other serpin type inhibitors, we analysed in this study the interaction of boar acrosin with other purified hum an serpins. Antithrombin III, plasminogen activator inhibitor-1, plasm inogen activator inhibitor-2 and alpha(1)-antitrypsin inhibited acrosi n activity. The following apparent k(app)s were calculated: Antithromb in III: 19.5x10(4)M(-1)s(-1), plasminogen activator inhibitor-1: 21.5x 10(4)M(-1)s(-1), plasminogen activator inhibitor-2: 3.3x10(4)M(-1)s(-1 ), alpha(1)-antitrypsin: 0.09x10(4)M(-1)s-(1). alpha(2)-azantiplasmin and heparin cofactor II did not inhibit acrosin. SDS-stable acrosin/se rpin complexes were only seen with antithrombin III; all other acrosin inhibitors were cleaved by the enzyme. As determined by enzyme-linked inmmunosorbent assays, the concentrations of protein C inhibitor, pla sminogen activator inhibitor-1, and plasminogen activator inhibitor-2 in individual seminal plasma samples from healthy donors were 5.33+/-0 .47 mu M, 88+/-24 pM and 163+/-17 pM (means+/-SD), respectively. The c oncentrations of antithrombin III in seminal plasma were less than or equal to 50 nM (semiquantitative immunoblotting). Therefore considerin g both, the k(app) calculated for the inhibition of boar acrosin in a purified system and the concentration of each serpin in seminal plasma , protein C inhibitor seems to be the best candidate to function as a physiological acrosin inhibitor in the male reproductive tract.