Pyruvate Dehydrogenase Activity in Rat Brain Mitochondria

1. The effects of phenylpyruvate, a metabolite produced in phenylketonuria, on the pyruvate dehydrogenase-complex activity were investigated in rat brain mitochondria. 2. Pyruvate dehydrogenase activity was measured by two methods, one measuring the release of 14CO2 from [1-1"C]pyruvate and the other measuring the acetyl-CoA formed by means of the coupling enzyme, pigeon liver arylamine acetyltransferase (EC 2.3.1.5). In neither case was there significant inhibition of the pyruvate dehydrogenase complex by phenylpyruvate at concentrations below 2mM. 3. However, phenylpyruvate acted as a classical competitive inhibitor of the coupling enzyme arylamine acetyltransferase, with a K, of 100/uM. 4. It was concluded that the inhibition ofpyruvate dehydrogenase by phenylpyruvate is unlikely to be a primary enzyme defect in phenylketonuria.