No abstract

that the rate of formation of FADH by dihydrolipoic acids is the rate-determining step in the catalytic reduction ofdiphosphopyridine nucleotide by dihydrolipoic acids. Similarly, the rate of reoxidation of FADH' by oxidized lipoic acids is the rate-determining step in the catalytic oxidation of reduced diphosphopyridine nucleotide by oxidized lipoic acids. The results of the rate experiments and titration experiments show that in the overall two-electron oxidation-reduction reaction catalysed by the enzyme the flavinadenine dinucleotide is reduced to FADHand reoxidized only once per catalytic cycle. The relation of this finding to the reaction mechanism of the enzyme is discussed.We are indebted to Dr I. C. Gunsalus and Dr L. J. Reed for generous gifts of DL-lipoic acid and lipoyl derivatives and to Miss M. Hamshaw for the preparation of large amounts of enzyme. One of us (C.V.