Inhibition of 5-aminolevulinic acid dehydratase activity by gabaculine

Abstract

Gabaculine (GAB, 3-amino 2,3-dihydroxybenzoic acid) treatment of developing radish seedling cotyledons induced a rapid decrease in 5-aminolevulinate dehydratase (EC 4.2.1.24) (5-ALAD) activity, which was more pronounced for light-grown seedlings. Some biochemical properties of 5-ALAD isolated from GAB-treated tissues were studied. No major difference was observed, as compared to the control, except for an increased K m (0.88–1.83 mM). The apparent K m of control enzyme was also increased by addition of aliquots of deproteinized extracts from treated tissues, which resulted in a decrease of the enzymatic activity in vitro. Analysis of these extracts after 2,3-diaminonaphthalene derivatization, along with specific enzymatic measurements, showed a net accumulation of (at least) 2-oxoglutarate and oxaloacetate, the former being known as a competitive inhibitor of 5-ALAD activity in vitro. It was concluded that the inhibition of 5-ALAD was the indirect consequence of gabaculine interaction with transaminating reactions. However, as 2-oxoglutarate is also observed in non-treated tissue extracts, but in smaller amounts, the role of this metabolic intermediate in the control of chlorophyll synthesis could be questioned.