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Abstract 1. Kinetic data concerning the substrate and product inhibition by pyruvate and l-lactate have been obtained with the M4 and H4 fractions separated from crystalline rabbit muscle lactic dehydrogenase. 2. The studies indicate that the product inhibition by l-lactate or by pyruvate is predominantly noncompetitive for both the M4 and H4 isozymes. Ki values for product inhibition by l-lactate were 130 mm for the M4 isozyme and 26.0 mm for the H4 isozyme. Ki values for product inhibition by pyruvate were 0.28 mm for the M4 isozyme and 0.18 mm for the H4 isozyme. 3. At the concentrations of pyruvate and l-lactate reported by others in contracting muscle, substrate inhibition does not occur to any significant extent under our experimental conditions. However, there is a marked difference in the extent of product inhibition by l-lactate of pyruvate reduction for the two isozymes, but very little difference in the product inhibition by pyruvate of l-lactate oxidation.
Stambaugh et al. (Fri,) studied this question.