pH dependence of tritium exchange with the C-2 protons of the histidines in bovine trypsin.

At pH 8.9 and 37 degrees C the half-times for tritium exchange with the C-2 protons of the histidines of trypsin are 73 days for His-57, and greater than 1000 days for His-40 and His-91. These half-times are much longer than the half-life of exchange for the C-2 proton of free histidine (2.8 days at pD 8.2), and longer than any previously reported half-time of exchange at pH greater than 8. These very low rates of exchange are discussed with reference to the refined structure of trypsin. The tritium exchange of His-57 depends on an apparent pKa of 6.6. This pKa may represent the pKa of the imidazole of His-57 in an inactive conformation of the enzyme.