The interaction of energy and electron transfer reactions in mitochondria. IV. The pathway of electron transfer.

Two types of reaction have been described in which a reversal of electron transfer may occur: (a) energy-linked reduction of pyridine nucleotide in the presence of a flavin-linked substrate (I) and (b) energy-linked oxidation of cytochrome and reduction of pyridine nucleotide (2). In the latter, the presence of both oxidized and reduced forms of the respiratory carriers suggests that the system may not require additional reducing equivalents from an external substrate, provided an initial state is achieved in which some carriers are reduced and some are oxidized. In this reaction, electron transfer from oxidized carriers to pyridine nucleotide may suffice. The first reaction, however, is usually observed only in the presence of oxidized carriers, and therefore electrons must be transferred into the respiratory chain from an external substrate to provide the necessary reducing equivalents for reduction of pyridine nucleotide. This paper considers the pathway of electron transfer by which these reducing equivalents reach pyridine nucleotide. It has been shown that electrons donated from succinate to its dehydrogenase are required by this reaction (3), and a working experimental hypothesis is that succinate transfers its reducing equivalents to pyridine nucleotide in an energy-linked reaction involving certain carriers of the respiratory chain. Kinetic and inhibitor studies are performed to determine which components are so involved.