Inhibition of Reduced Diphosphopyridine Nucleotide Oxidation by Ammonia

Abstract

THE toxic responses of plants to ammonia supplied as anhydrous or aqueous ammonia or ammonium salts are well documented^1–3; but relatively little is known regarding the means by which toxicity to either plant or animal cells is brought about⁴. In the course of an investigation of this problem it was found that ammonia strongly inhibits the exogenous utilization of glucose, pyruvate, citrate, α-ketoglutarate, succinate and malate by a number of intact tissues and particulate preparations from plant sources. Of the Krebs' cycle substrates, succinate was inhibited less by ammonia than the others used. This partial resistance of succinate oxidation to ammonia inhibition pointed toward a possible effect on diphosphopyridine nucleotide since this does not serve as a co-factor for succinate oxidation. To investigate this question further, studies were made of the effect of ammonia on the reduction and oxidation of diphosphopyridine nucleotide in homogenates of red beet-root, using the 340 mµ absorption peak of the reduced diphosphopyridine nucleotide. Homogenates of the fresh root of red beet (Beta vulgaris) were prepared in the cold by hand grinding and centrifugation. The suspending medium was 0.2 M tris (hydroxymethyl) aminomethane buffer at pH 8.5 containing 1.0 M sucrose and 5 × 10⁻³ M ethylenediamine tetra acetic acid.