Abstract
Electron spin resonance studies on soluble succinic dehydrogenase from beef heart under low partial pressures of oxygen arc reported, and the results of these studies are compared with related studies carried out under aerobic conditions. The electron spin resonance properties of succinic dehydrogenase at 30 ° are found to be substantially different when observed under anerobic conditions or at low partial pressures of oxygen, as compared with aerobic systems. The differences are ascribed to the reversible binding of oxygen to one or more species of enzyme and are reflected in the ways in which succinate and other dicarboxylic acids influence radical concentration. Irreversible reactions between enzyme and oxygen, such as autoxidation, appear to proceed too slowly to be of importance.
Under anerobic conditions succinate can convert 50% or more of the enzyme to a radical form. Fumarate represses radical yield more effectively than does oxaloacetate. Malonate and methylfumarate enhance radical yield at low concentrations except when succinate is present in saturating amounts. Under aerobic conditions, both succinate and fumarate are required for a maximum radical yield of about 25%, and oxaloacetate is extremely effective in decreasing radical yield. Malonate represses radical concentration, but its action appears generally to be biphasic.