ABSTRACT
Trypsin is a major member of the serine proteases, which constitute a large family of biologically important enzymes structurally and functionally con served from bacteria to mammals. Trypsin consists of a single peptide chain with molecular weight typically 24 kDa. It is synthesized in the pancreas and secreted as an inactive precursor, trypsinogen, from the pancreatic acinar cells together with chymotrypsinogen and proelastase (1,2). This evidence was ob served not only in mammals but also in fish as demontrated in chum salmon, Oncorhynchus keta (3), in catfish, Parasilurus asotus (4), and in eel, Anguilla japonica (5). Trypsinogen is activated by enteropeptidase in the small intes tine, by specific cleavage of a unique single peptide-bond between a con served positively charged residue, usually lysine, and isoleucine. Its N-terminal peptide of six to eight residues, depending on species, is removed (6), and the isoleucine residue at the new N-terminus bends inwards to make several internal contacts forming the catalytically active trypsin (7, 8). The small amount of trypsin activates more trypsinogen and all other pancreatic zymogens: chymotrypsinogen, procarboxypeptidase, and proelastase (1). The active proteolytic enzymes differ markedly in their degree of substrate speci-
Table 1 Pancreatic Proteases
Inactive enzyme Active enzyme Site of cleavage
Trypsinogen Trypsin Carboxyl side of Lys and Arg Chymotrypsinogen Chymotrypsin Carboxyl side of Phe, Trp, Tyr and of
large hydrophobic residues such as Met Procarboxypeptidase Carboxypeptidase Carboxyl-terminal residue with an
aromatic or a bulky aliphatic side chain Proelastase Elastase Carboxyl side of residue with small
hydrophobic side chain such as Ala
ficity, which is usually high and sometimes virtually absolute (Table 1). X-ray studies have shown that these different specificities are due to quite small dif ferences in the binding site. Most proteolytic enzymes also catalyze a differ ent but related reaction, namely, the hydrolysis of an ester bond. The principal protease that is active only in an acid environment of gastric juice is pepsin, while the main proteases that are active in an alkaline medium of the intestine are trypsin and chymotrypsin. Proteins are broken down by these principal proteases into the polypeptides, which are further broken down to amino acids and peptides of smaller molecular size by intestinal peptidases and pancreatic carboxypeptidases. Proteolytic enzymes affect food digestion and supply of amino acid precursors for protein synthesis. Since trypsin is the key enzyme for activating all other pancreatic zymogens, it may play a major role in regu lating protein digestion and affect the availability of nutrients for synthesis and growth. Several isoforms of trypsin have been reported in fishes, and the results are summarized in Table 2. Trypsin isozymes are believed to possess differences in their catalytic efficiency (kcJ K m) as demonstrated in cod, Gadus morhua, by Asgeirsson et al. (18). Isoforms with major differences in the distribution of charged amino acids may have different substrate-binding preferences (15, 22).
