COMPARISON BETWEEN THIFENSULFURON METHYL-INDUCED INACTIVATION OF BARLEY ACETOHYDROXYACID SYNTHASE AND ESCHERICHIA-COLI ACETOHYDROXYACID SYNTHASE ISOZYME-II

Thifensulfuron methyl, a sulfonylurea herbicide, inhibited acetohydrox yacid synthase (AHAS) from Escherichia coli (isozyme II) and barley in a similar, time-dependent, manner. This was modeled in terms of a slo w first-order transition (0.036 and 0.048 min(-1), respectively) from an initial, relatively weak complex of the enzyme with inhibitor to a final, more potently inhibited form. AHAS from both barley and E. coli appeared permanently inactivated. In neither case was activity recove red following removal of the inhibitor by gel filtration or dilution. However, removal of the inhibitor by precipitation with ammonium sulfa te yielded a quite different and surprising result. Bacterial AHAS rec overed its full activity, whereas the barley enzyme recovered none. Th us, inhibitor-induced inactivation of barley AHAS corresponds to a muc h less readily reversed change than in the case of bacterial enzyme. E xperiments were carried out to explore whether enzyme inactivation req uired factors in addition to the inhibitor. Contrary to reports elsewh ere, pyruvate (and therefore catalytic turnover) was not required. In the case of the bacterial enzyme, thiamine pyrophosphate (TPP)-Mg2+ wa s an absolute requirement. TPP bound some 220-fold more strongly to ba rley than to bacterial AHAS and, correspondingly, was also released a great deal more slowly (a half-time for dissociation of the enzyme:cof actor complex of similar to 10 days compared to similar to 1 hr). Here we suggest a mechanism whereby these differences in affinity for TPP- Mg2+ might underpin the apparent differences in the reversibility of s ulfonylurea-induced inactivation of AHAS from the plant and bacterial sources. (C) 1996 Academic Press.