The intracellular distribution of pentose cycle activity in rabbit kidney and liver.

Horecker (1) has demonstrated the occurrence of a cyclic mechanism in yeast and animal tissues which can account for the oxidation of carbohydrate to carbon dioxide and water without participation of triosephosphate dehydrogenase or the enzymes of the tricarboxylic acid cycle. The term “pentose cycle” has been suggested (2) for this series of reactions previously called the “shunt” or “oxidative pathway,” since the reactions occur in a cyclic manner and involve pentoses as key compounds. Knowledge of the presence of this cycle has been extended to include plants (3), bacteria (4), insects (5), and the wheat smut fungus (6). This cycle might function as an energy source and also as a means by which ribose, an important constituent of nucleic acids and certain coenzymes, can be synthesized or catabolized. Several workers (7, 8) have demonstrated the nearly exclusive location within the mitochondria of many oxidative enzymes, particularly those involved in the tricarboxylic acid cycle, various electron carrier systems, and oxidative phosphorylation. The glycolytic enzymes for the anaerobic conversion of carbohydrate have been shown to be localized in the soluble portions of the cell (9). Since the reactions of the pentose cycle may conceivably be associated with either or several of these activities, their behavior in the centrifugal field seemed of interest. Several experimenters (W-14) have compared the quantitative contribution of the pentose cycle and the glycolytic scheme (together with the Krebs cycle) to the oxidation of carbohydrate. Organisms which contain little or no Krebs cycle activity may obtain much of their energy and assimilate carbon by such a mechanism, as suggested by Scott and Cohen (15) for