Acid protease and its proenzyme from human seminal plasma.

An acid protease, with an optimum pH of 2.5, exists in seminal plasma in a proenzyme form. In acidic pH, the proenzyme is converted into the active form, resulting in the release of small molecular weight peptide. The extent and rate of proenzyme-active enzyme conversion is absolutely dependent on pH. Between pH 5 and 2, as the pH is lowered, the extent of conversion increases and reaches a maximum between pH 3 and 2. The kinetics of activation shift from first-order between pH 2 and 4 to more complexity with a lag period between pH 4.5 and 5. Under physiological conditions, the proenzyme might be activated by coming into contact with acidic vaginal fluid during ejaculation. The acid protease can hydrolyze the cervical mucus protein.