Abstract
Phosphoenolpyruvate carboxykinase activity in crude extracts of [rat] muscle has frequently been determined by using a continuous spectrophotometric method, which grossly overestimates enzyme activity. NADH oxidation attributed to phosphoenolpyruvate carboxykinase activity in the assay is due to lactate production. Under the normal assay conditions, Na+ ions stimulate pyruvate kinase, providing pyruvate for lactate formation by lactate dehydrogenase and sufficiently to account for most of the observed NADH oxidation.

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