Temperature-dependent changes in the activity and tryptic susceptibility of membrane-bound transhydrogenase

Beef heart mitochondrial pyridine nucleotide transhydrogenase undergoes a 2-fold changes in apparent activation energy between 5 degrees and 40 degrees C, from 30 kcal/mol at the lower end to 15 kcal/mol at the upper end of this range. In order to determine whether changes in the structural domain of transhydrogenase accompany the large change in activation energy, the degree of proteolytic inactivation of membrane-bound transhydrogenase was monitored as a function of temperature. Changes in the susceptibility of transhydrogenase to proteolytic inactivation over the 5 degrees -40 degrees C interval correlate well with the observed alteration in activation energies. No temperature-dependent change in either apparent activation energy or susceptibility to proteolysis were observed in detergent-solubilized transhydrogenase. Interactions between transhydrogenase and its microenvironment in the mitochondrial membrane may be an important factor in determining its activity.