[73] Metabolism of phenylalanine (Achromobacter eurydice)

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This chapter describes the metabolism of phenylalanine. A bacterium capable of growing on L-phenylalanine as the sole source of carbon is isolated from soil by enrichment culture technique. The cell-free extract of microorganism––Achromobacter eurydice––metabolizes L-phenylalanine to phenylacetate. Phenylacetate is not degraded by cell-free extracts. The enzymes involved in the metabolism of L-phenylalanine are inducible and appear only when cells are grown on L-phenylalanine or on L-tryptophan as the sole source of carbon. Phenylalanine aminotransferase is assayed by measuring the rate of formation of phenylpyruvate as its enol-borate complex, which absorbs at 300 mμ. This enzyme shows broad substrate specificity for both the amino donor and the keto acid receptor. L-tryptophan, L-tyrosine, L-kynurenine, and L-aspartate can replace L-phenylalanine in the transamination reaction with α-ketoglutarate. In the reaction between L-tyrosine and phenylpyruvate, the activity is measured by the formation of p-hydroxyphenylpyruvate. A unit of phenylalanine aminotransferase is referred as the amount of enzyme that causes the formation of 1 micromole of phenylpyruvate in 1 minute at 37°. Specific activity is expressed as the number of units per milligram of protein.

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