Mechanisms of decaborane toxicity

In the presence of water, decaborane is rapidly transformed into one or more polar intermediate products that are eventually degraded to boric acid. The intermediate, but not decaborane or boric acid, reduces phosphomolybdic acid and inhibits glutamic-oxaloacetic transaminase. The inhibition of transaminase activity is kinetically irreversible when the ratio of apoenzyme to pyridoxal phosphate is kept constant, but is partially reversed when excess pyridoxal phosphate is added. The inhibitory activity disappears when the intermediate is neutralized with pyridoxal phosphate so that both the phosphomolybdate-reducing capacity of the intermediate and the spectral characteristics of the pyridoxal phosphate are abolished. Prior loading of mice with pyridoxine raises the LD50 of decaborane. It is concluded that toxicity of decaborane attendant upon inhibition of pyridoxal phosphate requiring enzymes is due not to decaborane per se, but to reduction of pyridoxal phosphate by a substance formed nonenzymically from decaborane in the presence of water.