Phenylalanine ammonia-lyase and cinnamic acid 4-hydroxylase: Product repression of the level of enzyme activity in potato tuber discs.

Exogenous supplies of phenylalanne, cinnamic acid and p-coumaric acid can inhibit the appearance of phenylalanine ammonia-lyase (PAL, E.C. 4.3.1.5) activity in potato tuber discs, and exogenous supplies of cinnamic acid and p-coumaric acid can inhibit the appearance of cinnamic acid 4-hydroxylase (CA4H, E.C. 1.14.13.11) activity. The time-courses of the inhibitory effects of (a) exogenous phenylalanine on the appearance of PAL activity and (b) exogenous cinnamic acid on the appearance of CA4H activity suggest that the respective substrates of the two enzymes have no inhibitory effects per se, but rather that metabolic derivatives are the active agents. Accordingly, cinnamic acid and p-coumaric acid, but not phenylalanine can inhibit the initial appearance of PAL activity. Similarly, p-coumaric acid but not cinnamic acid can inhibit the initial appearance of CA4H activity. The inhibitory effects of exogenous cinnamic acid and p-coumaric acid on the initial appearance of PAL activity are independent. These findings are discussed in relation to the control in vivo of the levels of activity of PAL and CA4H.