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Hydrolysis of a synthetic amide substrate by human complement c 1 esterase


, : Hydrolysis of a synthetic amide substrate by human complement c 1 esterase. Journal of Immunology 119(1): 19-25

C1 [complement component 1] esterase (C1.hivin.s), the enzymatically active form of the C1s subunit, activates the classical complement pathway by limited proteolysis of C2 and C4. In searching for chromogenic amide substrates which could reflect the physiologic function of C1.hivin.s, it was found that preparations of C1.hivin.s slowly hydrolyzed N-benzoyl-L-phenylalanyl-L-valyl-L-arginine p-nitroanilide (BzPheValArgNa). Chromatographic, electrophoretic and inhibition studies confirmed that the BzPheValArgNA-hydrolyzing activity of these preparations was attributable to C1.hivin.s. Although C1.hivin.s is known to cleave a number of N-acylated .alpha.-amino acid esters, no synthetic amide substrate was reported previously for this enzyme. Kinetic analysis of the hydrolysis of BzPheValArgNA by C1.hivin.s yielded an apparent Km of 0.36 mM and a Kcat [catalytic constant] of 5.7 min-1. Enzymatic activity toward BzPheValArgNA appeared to be controlled by 2 ionizable groups with pK of approximately 6.5 and 8.9. In view of the sequence homology between C1 esterase and the pancreatic serine proteases, it is suggested that these groups correspond respectively to the .beta.-carboxyl group of aspartic acid 102 and the .alpha.-amino group of isoleucine 16 in .alpha.-chymotrypsin. Although C1.hivin.s hydrolyzed BzPheValArgNA, it did not cleave N-benzoyl-DL-arginine p-nitroanilide. Substrate binding at the subsites occupied by the valyl and/or phenylalanyl residues is probably critical to the amidase activity of C1.hivin.s.

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