Control of lactate oxidation in fish hearts by lactate oxidase activity

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Abstract
Isolated hearts of ocean pout (Macrozoarces americanus) and sea raven (Hemitripterus americanus) were perfused with media containing [14C]lactate or pyruvate and the rate of 14CO2 production was monitored. Increases in exogenous lactate concentration resulted in increases in the rate of lactate metabolism. Under comparable perfusion conditions the rate of decarboxylation of pyruvate was three- to four-fold higher than that of lactate. This finding suggests that lactate oxidation was being limited by lactate oxidase. LDH was purified and the Km values for lactate and pyruvate assessed under conditions of saturating cofactor concentration. Both hearts had a muscle type LDH on the basis of Km (pyruvate). Lactate oxidase from ocean pout and sea raven heart displayed Km values of 25 and 20 mM for lactate, respectively. The Km values were well above the presumptive intracellular level of lactate in the perfused hearts. Considered together, the perfusion and isolated enzyme studies show that the catabolism of exogenous lactate is limited by the reaction catalyzed by lactate oxidase.

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