We have shown that 3-nitropropionate, an isoelectronic analogue of succinate, is a suicide inactivator of succinate dehydrogenase [succinate:(acceptor) However, the precise mechanism of inactivation, as well as mechanistic extrapolations to the oxidation of succinate, must await the elucidation of the structure of the modified enzyme. We can now explain the toxicity of plants such as Indigofera endecaphylla for mammals and fowl as being due to the irreversible blockage of the Krebs cycle by 3-nitropropionate carbanion.Nitroalkanes are good substrates for the flavoenzymes D-amino acid oxidase, glucose oxidase, and L-amino acid oxidase (1-5). The nitroalkane carbanions are many orders of magnitude more reactive than their conjugate carbon acids and, in one case (6), even more reactive than the best physiological substrates (e.g., nitroethane anion compared to glucose with glucose oxidase). Most importantly, and in contrast to any physiological substrate, the chemical mechanism of the enzymic oxidation of nitroalkanes is understood in great detail. For D-amino acid oxidase and nitroethane, it was shown (2) that the substrate carbanion attacks the flavin at N-5 in the rate-determining step to form a covalent adduct (pathway 1 in Eq. 1) from which nitrite is eliminated rapidly to form a highly reactive N-5 iminium adduct. In the normal catalytic cycle (pathway 2 in Eq. 1) the iminium is rapidly hydrated to form an N-5 carbinolamine from which reduced flavin is also rapidly eliminated to form the product acetaldehyde. However, the pivotal N-5 iminium intermediate, as would be expected, is highly reactive with a variety of nucleophiles in addition to H20 (2, 5). Those having unbonded electron pairs (such as NH3, NH20H, and mercap- Second, it occurred to us that, in contrast to D-amino acid oxidase and nitroethane (2)

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