The metabolism of pyruvate in pigeon liver

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Abstract
Pyruvate is rapidly oxidized by minced pigeon liver in a calcium-free phosphate saline medium, the O2/pyruvate being about 0.6 and the R.Q. about 1.4. Unlike the similar reaction in muscle, this is not inhibited by malonate or affected by fumarate. When pyruvate is present in high concs. (0.02-0.04 M) there is an accumulation of [alpha]-ketoglutarate. In the presence of malonate, acetoacetate and succinate also accumulate, and the pyruvate utilized is completely accounted for by these 3 substances. The addition of a C4-dicarboxylic acid is not required as it is in the similar reaction in muscle. Oxaloacetate and pyruvate react in liver, as in muscle, to form citrate and [alpha]-ketoglutarate. Added citrate is rapidly oxidized by liver. Thus the citric acid cycle can occur in pigeon liver. The relative importance of the citric acid cycle and the formation of [alpha]-ketoglutarate from pyruvate may be assessed from the relative rates of the two reactions. [alpha]-Ketoglutarate arises from oxaloacetate and pyruvate at least 4-5 times as rapidly as from pyruvate alone, and at low physiological concs., pyruvate may be metabolized largely by way of the citric acid cycle. The direct synthesis of [alpha]-ketoglutarate is the main reaction of pyruvate removal only when the conc. of pyruvate is high, or when no oxaloacetate is available, e.g. in malonate-poisoned tissue where the citric acid cycle is interrupted.

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