INHIBITION OF 5-AMINOLEVULINIC ACID DEHYDRATASE ACTIVITY BY GABACULINE

Gabaculine (GAB, 3-amino 2,3-dihydroxybenzoic acid) treatment of devel oping radish seedling cotyledons induced a rapid decrease in 5-aminole vulinate dehydratase (EC 4.2.1.24) (5-ALAD) activity, which was more p ronounced for light-grown seedlings. Some biochemical properties of 5- ALAD isolated from GAB-treated tissues were studied. No major differen ce 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 increas ed by addition of aliquots of deproteinized extracts from treated tiss ues, 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 accumulatio n of (at least) 2-oxoglutarate and oxaloacetate, the former being know n as a competitive inhibitor of 5-ALAD activity in vitro. It was concl uded that the inhibition of 5-ALAD was the indirect consequence of gab aculine interaction with transaminating reactions. However, as 2-oxogl utarate is also observed in non-treated tissue extracts, but in smalle r amounts, the role of this metabolic intermediate in the control of c hlorophyll synthesis could be questioned.