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L‐Lactate metabolism in potato tuber mitochondria

We investigated the metabolism of l‐lactate in mitochondria isolated from potato tubers grown and saved after harvest in the absence of any chemical agents. Immunologic analysis by western blot using goat polyclonal anti‐lactate dehydrogenase showed the existence of a mitochondrial lactate dehydrogenase, the activity of which could be measured photometrically only in mitochondria solubilized with Triton X‐100. The addition of l‐lactate to potato tuber mitochondria caused: (a) a minor reduction of intramitochondrial pyridine nucleotides, whose measured rate of change increased in the presence of the inhibitor of the alternative oxidase salicyl hydroxamic acid; (b) oxygen consumption not stimulated by ADP, but inhibited by salicyl hydroxamic acid; and (c) activation of the alternative oxidase as polarographically monitored in a manner prevented by oxamate, an l‐lactate dehydrogenase inhibitor. Potato tuber mitochondria were shown to swell in isosmotic solutions of ammonium l‐lactate in a stereospecific manner, thus showing that l‐lactate enters mitochondria by a proton‐compensated process. Externally added l‐lactate caused the appearance of pyruvate outside mitochondria, thus contributing to the oxidation of extramitochondrial NADH. The rate of pyruvate efflux showed a sigmoidal dependence on l‐lactate concentration and was inhibited by phenylsuccinate. Hence, potato tuber mitochondria possess a non‐energy‐competent l‐lactate/pyruvate shuttle. We maintain, therefore, that mitochondrial metabolism of l‐lactate plays a previously unsuspected role in the response of potato to hypoxic stress.