3-Nitropropionate, the toxic substance of Indigofera , is a suicide inactivator of succinate dehydrogenase

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Abstract
Apparently 3-nitropropionate, an isoelectronic analogue of succinate, is a suicide inactivator of succinate dehydrogenase [succinate:(acceptor)oxidoreductase, EC 1.3.99.1]. When rat liver mitochondria oxidize succinate in the presence of 3-nitropropionate carbanion, rate of O2 consumption decreases exponentially to a zero value. This pattern is duplicated by subsequent additions of mitochondria. The dependence of the apparent first-order rate constant for enzyme inhibition as well as the number of enzyme turnovers completed before inhibition, on concentrations of 3-nitropropionate carbanion and succinate are those expected for an active site-directed and irreversible inhibitor. The inactivated enzyme is not resuscitated by centrifugation and washing of the mitochondria, in contrast to malonate-treated enzyme, and malonate protects against irreversible inhibition. The inhibitor species is 3-nitropropionate carbanion, and no external nucleophile is required for inhibition. The respiratory rates, respiratory control ratios and ADP/O ratios obtained with NAD-linked substrates are unaffected by 3-nitropropionate carbanion. Apparently 3-nitropropionate carbanion is a highly specific, time-dependent and irreversible inhibitor of succinate dehydrogenase. By analogy with the reaction of nitroethane with D-amino acid oxidase, the carbanionic inhibitor forms a covalent N-5 adduct with the active site flavin. The precise mechanism of inactivation and mechanistic extrapolations to oxidation of succinate must await elucidation of the structure of the modified enzyme. The toxicity of plants as I. endecaphylla for mammals and fowl can be explained as due to irreversible blockage of the Krebs cycle by 3-nitropropionate carbanion.