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Effects of Hydroxamic Acids Isolated from Gramineae on Adenosine 5'-triphosphate Synthesis in Chloroplasts ¹

Facultad de Ciencias Básicas y Farmacéuticas, Universidad de Chile, Santiago, Casilla 653, Chile, Centro de Estudios Fotosintéticos y Bioquímicos (Consejo Nacional de Investigaciones Científicas y Técnicas, Fundación Miguel Lillo, and Universidad Nacional de Rosario), 2000 Rosario, Argentina

Two hydroxamic acids isolated from maize extracts, 2,4-dihydroxy-7-methoxy-1,4-(2H)-benzoxazin-3(4H)-one (DIMBOA) and the 2-O--D-glucopyranoside of DIMBOA, inhibit photophosphorylation by spinach chloroplasts. Both cyclic and noncyclic photophosphorylations were inhibited to the same extent. The concentrations producing 50% inhibition for DIMBOA and its glucoside were about 1 and 4 millimolar, respectively. These compounds inhibit coupled electron transport but do not affect basal or uncoupled electron transport. Both acids inhibit the ATPase activities of membrane-bound coupling factor 1 (CF₁) and of purified CF₁. On the basis of these results, it is concluded that DIMBOA and its glucoside act as energy transfer inhibitors of photophosphorylation.