METABOLISM OF PROPIONATE BY SHEEP LIVER. OXIDATION OF PROPIONATE BY HOMOGENATES

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Abstract
The rate and stability to aging of the metabolism of propionate by sheep-liver slices and sucrose homogenates were examined. Aging for up to 20 minutes at 37[degree] in the absence of added substrate had little effect with slices, whole homogenates or homogenates without the nuclear fraction. Metabolism of propionate by sucrose homogenates was confined to the mitochondrial fraction, but the mitochondrial supernatant (microsomes plus cell sap) stimulated propionate removal. The rate of propionate metabolism by liver slices was higher in a high potassium phosphate-bicarbonate medium [0[center dot]88([plus or minus]S E M 0[center dot]16) [mu]mole/mg of N/hr.] than in Krebs-Ringer bicarbonate medium [0[center dot]44 ([plus or minus]SE M 0-13) [mu]mole/mg of N/hour], Metabolism of propionate by sucrose homogenates freed from nuclei was dependent on the presence of O2, CO2 and adenosinetriphosphate ATP. Propionate removal was stimulated 250% by Mg2+ ions and 670% by cytochrome c. In the complete medium 2[center dot]39 ([plus or minus] S E M 0[center dot]15) [mu]moles of propionate were consumed/mg of N/hour. The ratio of O2 consumption to propionate utilization was sufficient to account for the complete oxidation of half the propionate consumed. The only products detected under these conditions were succinate, fumarate and malate. Propionate had no effect on the production of lactate from endogenous sources and did not itself give rise to lactate. Methylmalonate did not accumulate when propionate was metabolized and was not oxidized. It was detected as an intermediate in the conversion of propionyl-CoA into succinate. The rate of this reaction sequence was adequate to account for the rate of propionate metabolism by sucrose homogenates or slices, provided that the rate of formation of propionyl-CoA was not limiting. The methylmalonate pathway was predominantly a mitochondrial function. The metabolism of propionate appeared to be dependent on active oxidative phosphorylatioa.