The metabolism of aromatic compounds in higher plants. IV. Purification and properties of the phenylalanine deaminase of Hordeum vulgare.

The biosynthesis of certain benzenoid compounds found in plants, such as the hydroxycinnamic acids, lignin, flavonoids, and coumarins, has recently been actively investigated. In his review (1) on the biosynthesis of these secondary plant products, Neish points out that the results of tracer experiments show the hydroxycinnamic acids to be important precursors in the formation of lignin and other secondary plant products. More specifically, in their studies on lignification in intact plants, Neish et al. (2-6) have demonstrated that both n-phenylalanine and cinnamic acid are good precursors of lignin or of phenylpropanoid units that can be incorporated into lignin in monocotyledons and dicotyledons. In addition, they have found that monocotyledons but not dicotyledons can readily convert n-tyrosine to lignin (2, 3, 5). From the results of experiments in which radioactive n-phenylalanine and cinnamic acid were tested as precursors of lignin (2, 3), Brown et al. (6) postulated that L-phenylalanine underwent conversion of cinnamic acid through a sequence of reactions involving transamination, reduction, and dehydration. From other experiments with monocotyledons (2, 3), it appeared that L-tyrosine was converted to p-coumaric acid (p-hydroxycinnamic acid) by analogous reactions (6). However, Neish recognized that the results of the above experiments did not rule out the formation of cinnamic acid and p-coumaric acid by the direct deamination of the corresponding amino acids (1). In a recent publication on the biosynthesis of chlorogenic acid, Levy and Zucker (7) presented additional evidence for the conversion of n-phenylalanine to cinnamic acid in disks of potato tuber. The present report describes the isolation, partial purification, and characterization of phenylalanine deaminase, an enzyme from barley (Hordeum vulgare L. var. Sravat) that converts Lphenylalanine to cinnamic acid and ammonia (Reaction 1).