Microsomal triphosphopyridine nucleotide-cytochrome c reductase of liver.

The biological role of reduced triphosphopyridine nucleotide (TPNH) and the metabolic pathways of its hydrogen atom and electron appear to be fundamentally different from those of reduced diphosphopyridine nucleotide (DPNH). The latter coenzyme appears to be intimately involved in cellular adenosine triphosphate (ATP) production, and is oxidized by both a phosphorylating, antimycin-sensitive pathway, and a nonphosphorylating pathway involving reduction of cytochrome c either directly or through the mediation of cytochrome bg. TPNH, on the other hand, appears to be a major source of reducing power for biosynthesis, but it has been assigned no role in organized electron transport. ATP production from this coenzyme is difficult to demonstrate; its reducing power is either directly utilized in biosynthetic reactions, or transferred to cytochrome c via TPNHcytochrome c reductases without concomitant phosphorylation. Although these reductases have been known for some time (1, 2)) their biological role is but poorly understood, and this understanding is handicapped by lack of knowledge of intracellular localization of these enzymes. The initial isolation of TPNH-eytochrome c reductase from yeast (1) gave no clues as to its localization within the cell; the liver reductase obtained by Horecker (2) was purified from whole liver acetone powder, and although Horecker demonstrated that activity was associated with particles, the nature of these particles was not clearly established. The role of various cell fractions in metabolism of DPNH is better defined than it is for TPNH; the microsome was shown to be involved in DPNH metabolism with the isolation by Strittmatter and Velick (3) of microsomal cytochrome, designated cytochrome bs, as well as a DPNH-specific cytochrome bs reductase (4, 5). Since cytochrome b6 can reduce cytochrome c, these two enzymes summate as a microsomal DPNH-cytochrome c reductase. Furthermore, a particulate complex with DPNHcytochrome c reductase activity, purified go-fold over microsomes, and containing flavin, cytochrome bs, and nonheme iron in a ratio of 1:2 : 1, was obtained by Penn and Mackler (6),