The metabolism of short-chain fatty acids in the sheep. 4. The pathway of propionate metabolism in rumen epithelial tissue

Abstract

When (carboxy-C14) propionic acid was metabolized by sheep-rumen epithelial tissue, the greater part of the isotope appeared in CO2. Most or all of the remainder was found in the carboxyl group of the lactate formed; the specific activity of the lactate was much less than that of the propionate. When propionate was metabolized, CO2 was fixed into lactate. Relatively little CO2 was fixed when pyruvate or fumarate was metabolized. In the presence of malonate, succinate of high and similar relative specific activity was formed from propionate and CO2 when either was labeled. Under similar conditions succinate formed from pyruvate, fumarate, lactate or acrylate had relatively low specific activity. This is taken as evidence that the metabolism of propionate involves CO2 fixation to form succinate. There was negligible labeling of a succinate pool when labeled propionate was metabolized. Fumarate and malate were produced from succinate by rumen epithelium. Propionate suppressed ketone-body formation from pyruvate and lowered the total amount of pyruvate metabolized.