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Summary

Mitochondria prepared by standard methods from the sterile spadix of Arum maculatum show relatively poor coupling (ADP/O = 0.5–1.0, respiratory control ratio =1.5–2.8). This coupling is, however, crucial to the response elicited by the application of cyanide. Mitochondria oxidizing malate are inhibited by cyanide with an apparent Ki of 0.3 mM, but an infinitely high cyanide concentration would produce only 50–60% inhibition. With succinate, similar high concentrations of cyanide cause substantial (30–50%) stimulation of the rate of oxidation. When the mitochondria are completely uncoupled by the presence of dinitrophenol or by freezing and thawing, the application of cyanide causes a 12–40% stimulation of the rate of malate oxidation and an even greater increase in that of succinate oxidation. This stimulation by cyanide is not dependent on the nature of the cation supplied.

The inhibitor of cyanide-resistant respiration, mCLAM, shows an inhibition additive to that of cyanide with coupled mitochondria, with an apparent Ki of 12–40 μM and 2 maximal inhibition of 40–80%, but in the presence of DNP the addition of mCLAM completely eliminates the cyanide-sensitive oxidation of succinate or malate.

These results indicate the presence of two pathways for electron transport in Arum mitochondria, of which one, the cyanide-resistant pathway, is more readily available to the dehydrogenase of succinate than to that of malate and those of other substrates using DPN. The uncoupling of phosphorylation appears to permit electrons to flow more readily through the resistant pathway from either malate or succinate. The cyanide-sensitive pathway appears to be inhibited readily by mCLAM, and the fact that the stimulation of oxidation by KCN is abolished by mCLAM indicates that the resistant pathway is responsible for the increased flow of electrons in the presence of cyanide.