Comparative Studies on the States of Amino Acid Residues in Porcine and Bovine Trypsins

Abstract

The states of amino acid residues in porcine and bovine trypsins were compared by spectrophotometric titration of tyrosine residues and by using several reagents: β-naphthoquinone-4, 6-disulfonic acid (NQDS) and monochlorotrifluoroquinone (CFQ) for amino groups, H₂O₂-dioxane for tryptophan, glyoxal for arginine, diazo-l-H-tetra-zole (DHT) for histidine and tyrosine, and cyanuric fluoride for tyrosine. Almost all the amino groups in these trypsins [EC 3. 4. 4. 4] were modified uniformly with NQDS and 5-6 amino groups of bovine trypsin and 4-5 amino groups of porcine trypsin were modified with CFQ. Glyoxal modified two of the total four arginine residues in porcine trypsin and one of the two arginine residues in bovine trypsin. Three of the four histidine residues in porcine trypsin and two of the three histidine residues in bovine trypsin were bisazotized with DHT. Three of the four tryptophan residues in bovine trypsin and all the four tryptophan residues in porcine trypsin were oxidized with H₂O₂-dioxane. Of the eight tyrosine residues in porcine trypsin three residues had values of pK=10.2, m (the order of the sigmoid ionization curve)=1.0, three values of pK=10.5, m=1.0 and two of pK=12.2, m=3.0. The three residues with a pK value of 10.2 were further classified in terms of their reactivity with CyF into two more reactive and one less reactive residues. The two residues with the highest pK value of 12.2 were not bisazotized while the other six residues were completely bisazotized. These data were compared with the previous observations that the ten tyrosine residues in bovine trypsin were of two types: six which ionized rapidly and reacted with CyF and four which ionized slowly and did not react with CyF. The difference in stability between bovine and porcine trypsins is discussed on the basis of these data, and the participation of a serine residue in the activity of porcine trypsin is postulated.