Chymotrypsin isoenzymes in Atlantic cod; differences in kinetics and substrate specificity.

Two chymotrypsin isoenzymes, ChT1 and ChT2 from cod pyloric caeca showed different kinetics against both chromogenic peptide and proteinaceous substrates. The enzymes have similar kcat values but ChT1 had substantially lower values for KM compared to ChT2. The enzymes also differed in cleaving site specificity with the oxidised B-chain of bovine insulin as substrate. ChT1 exhibited broader cleavage specificity compared to ChT2. This difference is parallel to the difference between chymotrypsin C and the A and B isotypes in higher animals. Direct N-terminal amino acid sequence analysis of the isoenzymes revealed that the active forms consisted of two polypeptide chains. The A chains were 13 residues long and ChT1 and ChT2 differed in three positions. The Cod enzyme A-chains differed from the A-chains of bovine chymotrypsin in four and five positions, respectively. The N-terminal part of the B-chains of the cod enzymes were nearly identical to that of the bovine A and B isoenzymes.