Prostate-specific antigen (PSA) is one of the three most abundant prostatic-secreted proteins in human semen. It is a serine proteinase that, in its primary structure, manifests extensive similarities with that of the Arg-restricted glandular kallikrein-like proteinases. When isolated from semen by the addition of chromatography on aprotininSepharose to a previously described procedure, PSA displayed chymotrypsin-like activity and cleaved semenogelin and the semenogelin-related proteins in a rapid and characteristic pattern, but had no trypsin-like activity. About one third of the purified protein was found to be enzymatically inactive, due to c!eavage carboxy-terminal of Lys145. Active PSA formed SDS-stable complexes with a,-antichymotrypsin, a2-macroglobulin, and the a2-macroglobulin-analogue pregnancy zone protein. PSA formed inhibitory complexes with a,-antichymotrypsin at a molar ratio of 1 : 1, a reaction in which PSA cleaved the inhibitor in a position identical to that reported from the reaction between chymotrypsin and a1 -antichymotrypsin. The formation of stable complexes between PSA and a,-antichymotrypsin occurred at a much slower rate than that between chymotrypsin and a,-antichymotrypsin, and at a similar or slightly slower rate than that between PSA and a2-macroglobulin. When added to normal blood plasma in vitro, active PSA formed stable complexes both with a2-macroglobulin and a,-antichymotrypsin. This complex formation may be a crucial determinant of the turnover of active PSA in intercellular fluid or blood plasma in vivo.
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