ARABIDOPSIS DIHYDROPTEROATE SYNTHASE - GENERAL-PROPERTIES AND INHIBITION BY REACTION-PRODUCT AND SULFONAMIDES

Dihydropteroate synthase (DHPS) (EC 2.5.1.15) was extracted from leave s of Arabidopsis thaliana and purified 21-fold by ion-exchange chromat ography. This enzyme preparation was then characterized for several of its properties. Michaelis-Menten constants for the substrates, p-amin obenzoic acid and dihydropteridine diphosphate were estimated to be 2. 5 and 91 mu M, respectively. In an optimized assay, the reaction produ ct, dihydropteroic acid, competitively inhibited the enzyme activity w ith a K-l of 81 mu M. However, neither dihydrofolate nor tetrahydrofol ate, products further downstream in the biosynthetic pathway inhibited enzymatic activity. This appears to be the first report of product in hibition of DHPS from a higher plant. The relative inhibitory properti es of several sulfonamides, analogues of p-aminobenzoic acid, were als o examined. The substitutions on the amide nitrogen of the sulfonamide s influenced the degree of inhibition; thus Ij, values for the inhibit ion of the DHPS activity by sulfanilamide, sulfacetamide and sulfadiaz ine were estimated to be 18.6, 9.6 and 4.2 mu M, respectively. The com petitive pattern of inhibition was shown in experiments with sulfadiaz ine. (C) 1997 Elsevier Science Ltd.