Abstract
The substrate analogue systems, chlorosuccinate-chlorofumarate and methylsuccinate-methylfumarate, replaced the succinate-fumarate system in electron spin resonance studies of radical formation with succinic dehydrogenase (Singer's soluble preparation from beef heart). The results indicate that the functions E′0 30 ° and (pH 6.5–8.5) calculated for the enzyme are invariant with respect to the substrate systems used. The fact that individual differences among the substrate systems are not reflected in these functions is interpreted to support an equilibrium mechanism involving the obligatory oxidation-reduction of succinic dehydrogenase.