RAPID INHIBITION OF THE SPERM PROTEASE ACROSIN BY PROTEIN-C INHIBITOR

Heparin was found to be an allosteric modulator of the amidolytic activity of the protease acrosin. In the presence of saturating concentrations of heparin, there was a 4.9-fold decrease in the value of the Michaelis constant for the substrate D-Ile-Pro-Arg-p-nitroanilide and the value of k(cat) was 2.5-fold lower. Analysis of the data yielded a dissociation constant of 0.22 +/- 0.04 mu M for the heparin-acrosin complex. The presence of relatively high concentrations of protein C inhibitor in seminal plasma [Laurell, M., Christensson, A., Abrahamson, P., Stenflo, J., and Lilja, H. (1992) J. Clin. Invest. 89, 1094-1101] suggests that this serpin may be involved in the control of the activity of acrosin. Acrosin was found to be rapidly inhibited by protein C inhibitor with the association rate constant (k(ass)) for the formation of the complex being (2.41 +/- 0.03) x 10(5) M(-1) s(-1) The value of k(ass) showed a bell-shaped dependence on the concentration of heparin; it was maximal at concentrations of heparin between 0.08 and 3 mu M and decreased at lower and higher concentrations. At the optimal heparin concentration, the value of k(ass) for the acrosin-protein C inhibitor reaction was 230-fold higher ((5.6 +/- 0.1) x 10(7) M(-1) s(-1)) than in the absence of heparin. The results suggest that protein C inhibitor may be important in the physiological control of acrosin activity, particularly where the presence of heparin-like glycosaminoglycans would stimulate the acrosin-protein C inhibitor reaction.