Over a flowering season of 7 weeks, mitochondria from the spadices of Arum maculatum showed a progressive increase in the rate at which added malate was oxidized from less than 20 nmoles min−1 mg protein−1 to more than 700 nmoles min−1 mg protein−1. In the oxidation of succinate, α-ketoglutarate and other substrates, much smaller differences in rate were found.
Mitochondria oxidizing malate responded to the addition of high (3.8 HIM) concentrations of cyanide by stimulation when their initial rate of malate oxidation was low and by inhibition when it was high. Stimulation by KCN even with high concentrations did not increase the O2 uptake rate of young spadices to the level of mature ones.
Associated with this alteration in response to cyanide was a reverse response to ADP. The early, slowly respiring mitochondria which were stimulated by KCN were often inhibited by ADP while the mitochondria from later spadices which were inhibited by KCN were usually stimulated by ADP.
Similar variations in the relative rate of malate oxidation, response to cyanide and to ADP were observed in mitochondria prepared on the same day from spadices at different stages of development from the same location. In this case the change from young (β) to fully matured (ɛ) spadices approximately paralleled those from the early season to the end of the flowering period. The changes in malate metabolism of the spadix mitochondria were not solely related to stage of spadix development, however, since those from spadices at the same apparent developmental stage gathered over a 5-week period also showed an increase in the rate of malate oxidation and a change in response to cyanide and ADP.
Several types of evidence—effects of additional washing of the mitochondria, response to additions of oxalacetate and pyruvate, response to inhibitors of cytochrome oxidase other than KCN, assay of oxalacetate concentration of mitochondria, and application of an inhibitor of cyanide-resistant electron transport (mCLAM)—support the hypothesis that one of the factors in the mitochondria from young or early spadices which limits their ability to oxidize malate is oxalacetate within the mitochondria. As the spadix develops, mitochondrial oxalacetate is diminished or disappears. The stimulation of oxygen uptake in the early stages due to cyanide is thus thought to result from the reduction of the quantity of oxalacetate by cyanohydrin formation.
The inhibition by ADP is not clearly understood, but may result from the existence of other reactions which are subject to negative control by ADP and which have no relation to the role of ADP in oxidative phosphorylation or its coupling to electron transport.
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