THE OXIDATION OF REDUCED TRIPHOSPHOPYRIDINE NUCLEOTIDE AS MEDIATED BY THE TRANSHYDROGENASE REACTION AND ITS INHIBITION BY THYROXINE

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Abstract
A purified prep-aration from heart muscle which contains all the components of the cytochrome electron transmitter system is shown to contain transhydrogenase. In the presence of this enzyme preparation succinate and DPNH react rapidly with O2, but no reaction whatsoever occurs with TPNH, whether it is added as such or is generated by the glucose-6-phosphate dehydrogenase system. Oxidation of TPNH does occur if a small amount of DPN is added. This DPN-mediated oxidation of TPNH requires cytochrome c and is inhibited by cyanide and the naphthoquinone SN 5949. The pathway of electron flow is thus characteristic of that for the oxidation of DPNH or succinate by the same enzyme preparation. L-Thyroxine inhibits the oxidation of TPNH by this pathway, and some of the possible implications of this finding for the mode of action of thyroxine in vivo are discussed.