Activation and inhibition of mitochondrial transhydrogenase by metal ions

Mitochondrial transhydrogenase has been reported previously to be inhibited by high, rather non-physiological concentrations (in the range of 2-20 mM) of divalent cations. We show that the enzyme could be activated by low (from about 1 μM to 1 mM) concentrations of Ca2+ and Mg2+, which are within physiological range. These results bring in line the effects observed with mitochondrial enzyme to the findings with bacterial transhydrogenases. The activation of transhydrogenase by divalent cations is interpreted as an increase in affinity of the NADP(H)-binding site of the enzyme-NAD(H) complex. Reported effects of the metal ions could be important for the enzyme function in vivo.