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

J.F Blazyk,J.M Blazyk,C.M Kline

doi:10.1016/s0021-9258(19)70029-5

Abstract

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.

Highlights

Changes in the susceptibility of transhydrogenase to proteolytic inactivation over the 5"-40°C interval correlate well with the observed alteration in activation energies
Succinaotxeidase (Raison et al, 1971), F,ATPasea, ndP-hydroxybutyrate dehydrogenase (Lenaz etal., 1972) undergoa sharpchange in activation energyin the vicinity of 20°C
Sincethe molecular mechanism(s) of activation energy changes remains unclear, we examined the possibility that Pyridine nucleotide transhydrogenase (EC 1.6.1.1) is a thermally-inducedconformational changes in the enzyme may membrane-boundmitochondrial enzyme that catalyzes the accompany the changes in slope of the Arrhenius plot of a transfer of a hydride ion between NAD and NADP: membrane-bound enzyme

Summary

Introduction

Changes in the susceptibility of transhydrogenase to proteolytic inactivation over the 5"-40°C interval correlate well with the observed alteration in activation energies. Raison and Mc-Murchie (1974) suggested that two temperature-induced changes in the activation energy of mitochondrial succinate oxidase occur at 24" and 8"C, corresponding to the upper and lower extremes of a phase transition in the membrane lipids. The results indicate that the apparent activation energy of transhydrogenase changes dramaticallybetween 5" and 37"C, that this changeis dependent upon the integrityof the membrane, and that there may be a link between the activity andconformation of the enzyme in the mitochondrial membrane.

Results

Conclusion