Nicotinamide-adenine dinucleotide-linked “malic” enzyme in flight muscle of the tse-tse fly (Glossina) and other insects

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Abstract
A high activity of NAD-linked malic enzyme was found in homogenates of flight muscle of different species of tse-tse fly (Glossina). The activity was the same as, or higher than, that of malate dehydrogenase and > 20-fold that of NADP-linked malic enzyme. A similar enzyme was found in the flight muscle of all other insects investigated [G. austeni, Spodoptera exempta] but at much lower activities. A Ca2+-stimulated oxaloacetate decarboxylase activity was present in all insect flight-muscle preparations investigated, in constant proportion to the NAD-linked malic enzyme. A partial purification of the NAD-linked malic enzyme from Glossina was effected by DEAE-cellulose chromatography, which separated the enzyme from malate dehydrogenase and NADP-linked malic enzyme, but not from oxaloacetate decarboxylase. The intracellular localization of the NAD-linked malic enzyme was predominantly mitochondrial; latency studies suggested a localization in the mitochondrial matrix space. Studies on the partially purified enzyme demonstrated a pH optimum of 7.6-7.9. It required Mg2+ or Mn2+ for activity; Ca2+ was not effective. The maximum rate was the same with either cation, but the concentration of Mn2+ required was 100 times less than that of Mg2+. Activity with NADP was only 1-3% of that with NAD, unless very high (> 10 mM) concentrations of Mn2+ were present. The NAD-linked malic enzyme probably functions in the proline-oxidation pathway predominant in tse-tse fly flight muscle.