Pathways of hydrogen transport in the oxidation of extramitochondrial reduced diphosphopyridine nucleotide in flight muscle.

Carbohydrates, principally glycogen, trehalose, and glucose, are the energy reserves for the flight of flies. Glycogen deposits are found in the soluble cytoplasm of the muscle, and the initial reactions of glycogen, trehalose, or glucose catabolism are localized in the cytoplasm (1). During glycolysis, the first oxidative step occurs at the triose level and reduced coenzyme, reduced diphosphopyridine nucleotide (DPNH), is formed by glyceraldehyde phosphate dehydrogenase. The pathways in flight muscle whereby this extramitochondrial DPNH becomes oxidized, or the mechanism whereby hydrogen passes through the cytoplasmic-mitochondrial barrier, are examined in this paper. The various pathways which have been described (2-7) for oxidizing exogenous DPNH by flight muscle include : Pathway 1, the direct mitochondrial oxidation of exogenous DPNH; Pathway 2, the oxidation of DPNH by pyruvate catalyzed by the soluble cytoplasmic lactic dehydrogenase; Pathway 3, the oxidation of DPNH by dihydroxyacetone phosphate catalyzed by the soluble cytoplasmic a-glycerophosphate dehydrogenase; and Pathway 4, the oxidation of DPNH by oxaloacetate catalyzed by the soluble cytoplasmic malic dehydrogenase. The direct mitochondrial oxidation of extramitochondrial DPNH assumes no barrier for DPNH. Pathway 2 leads to the accumulation of lactic acid in the cytoplasmic compartment. On the other hand, Pathways 3 and 4, coupled with the respective mitochondrial oxidation of oc-glycerophosphate and malate, enable reducing equivalents to pass the cytoplasmic-mitochondrial barrier. These coupled reactions may be illustrated as