Abstract
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.
Highlights
The studies indicate that the product inhibition by L-lactate or by pyruvate is predominantly noncompetitive for both the Mq and Hq isozymes
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
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
Summary
Recent studies by Dawson et al (l), Cahn et al (2) and Kaplan and Cahn (3) indicate that these M and H types of lactic dehydrogenase subunits, which combine into tetramers to give five isozymes, have significantly different physiological roles. This concept is based on the demonstration that the H form of the enzyme is inhibited by excess pyruvate, whereas the M form maintains its activity at high pyruvate concentrations. In view of the possible significance of product inhibition of pyruvate reduction by these high concentrations of L-lactate, it seemed desirable to us to carry out these experiments to determine more precisely the effects of substrate and product inhibition on the purified rabbit muscle lactic dehydrogenase H, and R/I4isozymes
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