Dehydrogenase Activity and Respiration; a Quantitative Comparison

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Abstract
The maximum activities of succinic and malic dehydrogenase and of succinic, malic, and alpha-ketoglutaric "oxidase" have been determined by the Thunberg and Warburg methods, respectively. The activities of extracts from Avena coleoptiles and Pisum stem sections have then been compared with the observed maximum rates of respiration of the intact tissues, in order to determine whether the Krebs cycle may be the main route of respiration. Within the limitations of such comparisons it is concluded that the amounts of succinic and malic dehydrogenase are sufficient to allow all of the tissue respiration to proceed through the cycle; in the case of alpha-ketoglutaric acid, the amount of enzyme found is a little short of theoretical. The amount of malic dehydrogenase present in the mitochondrial fraction alone is more than sufficient for cycle operation. As information on other enzymes in the cycle is lacking, the evidence for the Krebs cycle as a main respiratory pathway remains incomplete.

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