Studies on the Oxidation of Succinic Acid by Cell-free Homogenates of Tetrahymena

The mammalian enzyme system which catalyzes the aerobic oxidation of succinic acid has been shown by Keilin and Hartree (1) to consist of succinic dehydrogenase and the complete cytochrome system. Although cytochrome oxidase is generally considered to be the universal catalyst of all respiring cells (2), it has been reported to be absent from, or present in small amounts in, several aerobic organisms. Pappenheimer and Hendee (3) have shown that diphtheria bacillus extracts, which actively oxidize succinate, possess only traces of cytochrome c and cytochrome oxidase. Kun and Abood (4) described an enzyme preparation from the endotoxin of Salmonella aertrycke which oxidized succinate aerobically but did not contain measurable cytochrome oxidase activity. Muscle homogenates of the helminth Ascaris lumbricoides were reported to be devoid of cytochrome oxidase activity but able to oxidize succinate in the presence of oxygen (5). It was concluded that the succinic dehydrogenase of Ascaris does not react with the cytochrome system and that possibly a flavoprotein is involved. In two other parasitic worms, whose respiration was inhibited by 2 X 1O-4 M cyanide, cytochrome c and cytochrome oxidase were either not detected or were present in small amounts. A number of papers have been summarized which relate to the absence of, or variation in, certain components of the cytochrome system of various organisms (6, 7). Recently, Ryley (8) demonstrated that homogenates of Tetrahymena pyriformis GL, in the presence of succinate, reduced methylene blue anaerobically, but were unable to oxidize this substrate aerobically. A manometric assay in which p-phenylenediamine was employed failed to detect the presence of cytochrome oxidase activity. Respiration of the ciliate was comparatively resistant to cyanide and carbon monoxide. In contrast to these findings, Seaman has shown that homogenates of T. pyriformis S oxidize succinate aerobically (9, 10) and reduce cytochrome c in the presence of succinate and azide (11). This paper presents some observations on the enzyme system which