Relative confribution of dehydrogenases to overall respiratory ETh activity in some marine organisms

Abstract

Respiration is an oxidation-reduction process in which the electron flux through the respiratory electron transfer system (ETS) is sustained by the action of different dehydrogenases. These enzymes, as parts of the ETS, oxidize natural substrates (succinate, NADH and NADPH) of the cells and use the reducing equivalents to activate ATP synthesis. We studied the relative contribution of the three main dehydrogenases to the overall ETh activity in some marine organisms. Each organism was analysed for the combined and separate activities of NADH, NADPH and succinate dehydrogenases. The ETS activity was measured as the ability of each organism to reduce the tetrazolium salt, INT, when supplied with their natural substrates. The results showed that (i) NADH dehydrogenase was generally the most active dehydrogenase in prokaryotic and eukaryotic cells; (ii) INT does not fully collect reducing equivalents from succinate through the succinate dehydrogenase; and (iii) the sum of the activities measured separately exceeds the combined activity when the three enzymes are measured together. We suggest that competition of the individual dehydrogenases for a common limiting electron acceptor, ubiquinone, may explain these observations.