Mechanisms of inhibition of phenylalanine ammonia-lyase by phenol inhibitors and phenol/glycine synergistic inhibitors

Phenylalanine ammonia-lyase (PAL) catalyzes the beta-elimination of ammonia from L-phenylalanine to trans-cinnamic acid. A study of inhibition of PAL by phenol, ortho-cresol, and meta-cresol gave mixed inhibition; para-cresol is not an inhibitor. The calculated values of K-i and alphaK(i) are phenol, K-i = 2.1 +/- 0.5 mM and alphaK(i) = 3.45 +/- 10.95 mM; ortho-cresol, K-i = 0.8 +/- 0.2 mM and alphaK(i) = 3.4 +/- 0.2 mM; meta-cresol, K-i = 2.85 +/- 0.15 mM and alphaK(i) = 18.5 +/- 1.5 mM. The synergistic inhibition of the same inhibitors with glycine showed a lack of inhibition with the para-cresol/glycine pair, while mixed inhibition was observed with the ortho-cresol/glycine pair (K-i=0.038 0.008 mM, alphaX(i)=0.13 +/- 0.04 mM) and phenol/glycine pair (K-i=0.014 0.003mM, alphaK(i)=0.058 0.01 M). The meta-cresol/glycine pair gave competitive inhibition (K-i = 0.36 +/- 0.076 mM). The strong synergistic inhibition observed implies that the inhibitors bind at the active site: in fact, the inhibitors used imitate the structure of the substrate. The order of synergistic inhibition is the same for the sites related to K-i and alphaK(i). These results are in agreement with the inhibitors entering two active sites located in two different subunits. (C) 2003 Elsevier Science (USA). All rights reserved.