Inhibition by malonate of succinic dehydrogenase in heart-muscle preparations

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Abstract
Using K ferricyanide as hydrogen acceptor, the Michaelis constant (Km) of succinic dehydrogenase in 6 different heart-muscle prepns. had values ranging from 2.5 x 10-4 [image] to 5.3 x 10-4 [image]. In the same series of expts., the inhibitory constant (Ki) of malonate had values ranging from 5.4 x 10-6 [image] to 9.8 x -10-6 [image]. The ratio of affinities of succinic dehydrogenase for malonate and succinate (Km/Ki), detd. directly, varied from 4.7 to 60, depending on the nature and concn. of the hydrogen acceptor. This variation can be explained by differences in the value of the rate constant (k[image]3) for the decomposition of the enzyme-substrate complex to give the products of the reaction. The true ratio of affinities of succinic dehydrogenase for malonate and succinate, in the absence of further reaction of succinate, is about 3. Values of the rate constants of the uninhibited reaction were calculated, and found to agree substantially with those previously reported.