Dual control of phenylalanine ammonia-lyase production and removal by its product cinnamic acid.

Abstract

l‐Phenylalanine ammonia‐lyase (EC 4.3.1.9, which catalyses the first reaction in the biosynthesis of plant phenylpropanoid products, undergoes a transient increase in activity in excised sections of pea (Pisum sativum) epicotyl tissue. Exogenous supplies of pathway intermediates inhibit the initial development of phenylalanine ammonia‐lyase activity and, if added at the time of high enzyme levels, cause a rapid decay in enzyme activity. The inhibitory effect of exogenous cinnamic acid, the immediate product of the enzyme, is expressed very rapidly (lag < 1 h) with the initial rate of decay of enzyme activity and the final steady‐state level of enzyme activity being dependent on the concentration of cinnamic acid.The response has been studied by density labelling in vivo with 2H from 2H2O with analysis of the equilibrium distribution of enzyme activity in high‐resolution KBr density gradients. Cinnamic acid, at a concentration of 1 mM completely inhibits further incorporation of label and also markedly stimulates the removal of pre‐existing, unlabelled enzyme. Inhibition of de novo production and stimulation of removal occur at concentrations of cinnarnic acid as low as 10 μM.The initial increase in enzyme activity reflects an increase in the rate of de novo enzyme production against a low background rate of enz:yme removal. The subsequent decay in enzyme activity is a result of a decline in production of the enzyme concomitant with a marked increase in the rate of removal of enzyme activity. In the presence of α‐ aminooxy‐β‐phenylpropionic acid, which specifically inhibits phenylalanine ammonia‐lyase and hence endogenous generation of cinnamic cid, high rates of enzyme production and low rates of enzyme removal are maintained leading to high levels of zyme activity in the later stages of enzyme induction. These data indicate the operation of dual feedback modulation of phenylalanine ammonia‐lyase in vivo following endogenous production of cinnamic acid. Such dual control by a product over both production and removal of an enzyme may be of general importance in slowly growing plant cells for the rapid removal of biosynthetic enzymes no longer required by the cell.