Evidence for an intrazymogen mechanism in the conversion of proacrosin into acrosin

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Abstract
The mechanism responsible for the spontaneous initiation of boar sperm proacrosin conversion into acrosin in vitro was studied and chacterized by using the highly effective inhibitor leupeptin. In the presence of excess leupeptin [(102-103) inhibition constant to acrosin], proacrosin spontaneously and completely converted into acrosin at pH 8. Only the initial enzyme product, m.alpha.-acrosin, was produced, and the rate of conversion was not affected when exogenous m.alpha.-acrosin was added to the reaction mixture. Excess leupeptin eliminated all conversion and degradative reactions requiring active acrosin. Kinetically, the conversion of proacrosin into m.alpha.-acrosin in the presence of excess leupeptin appeared first order. The observed half-life (t1/2 = 1.4 h) did not vary over a 10-fold range of leupeptin or initial proacrosin concentrations. Proacrosin can self-catalyze its own conversion into m.alpha.-acrosin by an intrazymogen mechanism.