Amino acid sequence of rat submaxillary tonin reveals similarities to serine proteases.

Tonin, an esteroprotease isolated from rat submaxillary gland, is a serine protease with trypsin- and chymotrypsin-like activity. The substrate specificity of tonin shows that it differs from kallikreins and is definitely not a renin-like enzyme or an angiotensin-converting enzyme. Tonin can produce directly the vasoactive peptide angiotensin II, from angiotensin I, angiotensinogen and the synthetic tetradecapeptide substrate of renin by cleavage of a Phe-His bond. It has also been found to cleave some Phe and Arg bonds in various substrates such as beta-lipotropin (beta-LPH), adrenocorticotropin (ACTH), pro-opiomelanocortin (POMC) and substance P. Here we describe the complete amino acid sequence of rat submaxillary gland, tonin. Comparison of the sequence of 219 amino acids with other serine proteases, particularly kallikreins, gamma-subunit of nerve growth factor (NGF) and the recently described gamma-renin, reveals extensive similarities. More interestingly, it also reveals the substitution of an Asp residue always found in the serine protease active site triad (Asp, His, Ser) by a Leu residue. This unusual substitution does not seem to affect the proteolytic activity of the enzyme.