INHIBITION OF ANTHOCYANIN SYNTHESIS AND PHENYLALANINE AMMONIA-LYASE ACTIVITY BY CO2+ IN LEAF-DISKS OF TERMINALIA-CATAPPA

Treatment with sucrose induced anthocyanin synthesis and phenylalanine ammonialyase (PAL, EC 4.3.1.5) activity in leaf disks of Indian almond (Terminalia catappa L. Duthie). Co2+, an inhibitor of ethylene biosynthesis, inhibited anthocyanin synthesis and PAL activity when given together with sucrose. Ethephon (an exogenous source of ethylene) given along with sucrose, promoted anthocyanin synthesis and PAL activity, but in the presence of Co2+ its effectiveness decreased. In an attempt to understand the inhibitory action of Co2+ in the presence of ethephon, the effect of Co2+ on PAL activity was studied in vitro. A kinetic study showed an uncompetitive type of inhibition of PAL by Co2+, which was not time dependent. Addition of 2-mercaptoethanol, cysteine or glutathione overcame the in vitro effect of Co2+, and 2-mercaptoethanol also restored the activity of PAL extracted from Co2+-treated leaf disks. It is suggested that sulfhydryl group(s) might be involved in the inactivation of PAL by Co2+. The effects of N-ethylmaleimide (NEM) and HgCl2 (other sulfhydryl reagents) were also studied. Both NEM and Hg2+ Competitively inhibited PAL activity in vitro, and the inhibition could be reversed by sulfhydryl compounds.