Acid proteases from species of Mucor. IV. Hydrogen-tritium exchange of Mucor miehei protease.

The kinetics of hydrogen-tritium exchange were studied in the range pH-3 for both the fully and partially tritiated protein. Exchange constants for an intermediate class and slow class of hydrogens were determined and found to give a parabolic curve characteristic of acid and base catalysis about the observed pHmin of 4.03. The anomalous rate retardation on the acid portion of the curve was attributed to electrostatic interactions which could be evaluated quantitatively from the titration data. Partial tritation and pH cross-over experiments indicated that the rank order was pH-independent thus eliminating the possiblitity of a major conformational change. Consequently, the data are most likely explicable in terms of restricted solvent accessibility.