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

Treatment with sucrose induced anthocyanin synthesis and phenylalanine ammonialyase (PAL, EC 4.3.1.5) activity in leaf disks of Indian almon d (Terminalia catappa L. Duthie). Co2+, an inhibitor of ethylene biosy nthesis, inhibited anthocyanin synthesis and PAL activity when given t ogether 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 etheph on, the effect of Co2+ on PAL activity was studied in vitro. A kinetic study showed an uncompetitive type of inhibition of PAL by Co2+, whic h was not time dependent. Addition of 2-mercaptoethanol, cysteine or g lutathione overcame the in vitro effect of Co2+, and 2-mercaptoethanol also restored the activity of PAL extracted from Co2+-treated leaf di sks. It is suggested that sulfhydryl group(s) might be involved in the inactivation of PAL by Co2+. The effects of N-ethylmaleimide (NEM) an d HgCl2 (other sulfhydryl reagents) were also studied. Both NEM and Hg 2+ Competitively inhibited PAL activity in vitro, and the inhibition c ould be reversed by sulfhydryl compounds.