Properties of phenylalanine ammonia-lyase extracted from Cucumis sativus hypocotyls

Abstract

Some of the in vitro properties of PAL from gherkin hypocotyls were investigated. No metal ion requirement for this enzyme could be demonstrated but there were indications that PAL was a sulphydryl enzyme. Kinetic analysis suggested that PAL exhibited negative homotropic cooperativity. Two K m values were determined, these were K m H, 2·9 × 10 −4 M and K m L, 4·3 × 10 −5 M. Strong inhibition of the enzyme was exerted by d-phenylalanine, trans-cinnamic acid, o-coumaric acid, gallic acid, quercetin and kaempferol. Kinetic studies on the inhibition patterns of these compounds established that d-phenylalanine linearized the curvilinear kinetics, trans-cinnamic acid and o-coumaric acid acted as competitive inhibitors whilst gallic acid, quercetin and kaempferol acted as mixed inhibitors. Using a number of desensitization techniques PAL was partially desensitized to inhibition by the mixed inhibitors. These results led to the conclusion that PAL may have a regulatory role in phenol, coumarin and flavonoid biosynthesis.