Human prostate-specific antigen: Structural and functional similarity with serine proteases

Abstract

The complete amino acid sequence of the prostate-specific antigen (PA) from human seminal plasma has been determined from analyses of the peptides generated by cyanogen bromide, hydroxylamine, endoproteinase Arg-C and Lys-C. The single polypeptide chain of PA contains 240-amino acid residues and has a calculated M(r) of 26,496. An N-linked carbohydrate side chain is predicted as asparagine-45, and O-linked carbohydrate side chains are possibly attached to serine-69, threonine-70, and serine-71. The primary structure of PA shows a high degree of sequence homology with other serine proteases of the kallikrein family. The active site residues of histidine, aspartic acid, and serine comprising the charge-relay system of typical serine proteases were found in similar positions in PA (histidine-41, aspartic acid-96, and serine-192). At pH 7.8, PA hydrolyzed insulin A and B chains, recombinant interleukin 2, and - to a lesser extent - gelatin, myoglobin, ovalbumin, and fibrinogen. The cleavage sites of these proteins by PA were chemically analyzed as the α-carboxyl side of some hydrophobic residues, tyrosine, leucine, valine, and phenylalanine, and of basic residues histidine, lysine, and arginine. The chymotrypsin-like activity of PA exhibited with the chromogenic substrate N-succinyl-L-alanyl-L-alanyl-L-proyl-L-phenylalanine p-nitroanilide yielded a specific activity of 9.21 μM per min per mg of PA and K(m) and k(cat) values of 15.3 mM and 0.075 s-1, respectively. 'Trypsin-like' activity of PA was also detected with Nα-benzoyl-DL-arginine p-nitroanilide and gave a specific activity of 1.98 μM per min per mg of PA. Protease inhibitors such as phenylmethylsulfonyl fluoride, diisopropyl fluorophosphate, L-1-tosylamido-2-phenylethyl chloromethyl ketone, aprotinin, leupeptin, soybean trypsin inhibitor as well as Zn2+ and spermidine were effective inhibitors of PA enzymatic activity.