Hepatic triphosphopyridine nucleotide-cytochrome c reductase: isolation, characterization, and kinetic studies.

has previously been isolated by Horecker (1) from an acetone powder of liver and characterized as a flavoprotein. It was initially suggested that this enzyme was localized in hepatic mitochondria. However, subsequent studies have revealed that triphosphopyridine nucleotide-cytochrome c reductase activity is also present in hepatic microsomes (a), and it has been reported that the properties of the mitochondrial and microsomal enzymes are not identical (3). Inasmuch as cytochrome c is localized within the mitochondria (4), the problem of identifying the physiQlogica1 electron acceptor for the microsomal enzyme has been puzzling. We now wish to report evidence that hepatic TPN-cytochrome c reductase is most probably localized exclusively within the microsomes, and that the enzyme isolated from this source closely resembles that previously isolated by Horecker (1). illoreover, evidence is presented which suggests that this enzyme participates in a microsomal elect,ron transport system which does not include cytochrome c. In addition, kinetic evidence has been obtained which allows certain conclusions to be drawn concerning the mechanism of catalysis by this enzyme.