Effect of phenylpyruvate on enzymes involved in fatty acid synthesis in rat brain.

1. The activities of, and the effects of phenylpyruvate on, citrate synthase (EC 4.1.3.7), acetyl-CoA carboxylase (EC 6.4.1.2) and fatty acid synthetase derived from the brains of 14-day-old and adult rats were investigated. 2. The brain citrate synthase from 14-day-old rats had a K(m) for oxaloacetate of 2.38mum and for acetyl-CoA of 16.9mum, and a V(max.) of 838nmol of acetyl-CoA incorporation/min per mg of mitochondrial protein. From adult rat brain this enzyme had a K(m) for oxaloacetate of 2.5mum and for acetyl-CoA of 16.6mum and a V(max.) of 1070nmol of acetyl-CoA incorporated/min per mg of mitochondrial protein. Phenylpyruvate inhibited the enzyme from adult and young rat brains in a competitive fashion with respect to acetyl-CoA, with a K(i) of 700mum. 3. The brain acetyl-CoA carboxylase from 14-day-old rats had a K(m) for acetyl-CoA of 21mum and a V(max.) of 0.248nmol/min per mg of protein, and from adult rats a K(m) for acetyl-CoA of 21mum and a V(max.) of 0.173nmol/min per mg of protein. The enzyme from young and adult rats required citrate (K(a)=3mm) for activation and were inhibited non-competitively by phenylpyruvate, with a K(i) of 10mm. 4. The brain fatty acid synthetase from 14-day-old rats had a K(m) for acetyl-CoA of 7.58mum and a V(max.) of 1.1 nmol of malonyl-CoA incorporated/min per mg of protein, and from adult rats a K(m) for acetyl-CoA of 4.9mum and a V(max.) of 0.48nmol of malonyl-CoA incorporated/min per mg of protein. Phenylpyruvate acted as a competitive inhibitor with respect to acetyl-CoA with a K(i) of 250mum for the enzyme from 14-day-old rats. 5. These results are discussed with respect to phenylketonuria, and it is suggested that the inhibition of the brain fatty acid synthetase and possibly the citrate synthetase by phenylpyruvate could explain the defective myelination characteristic of this condition.