Metabolism of N-Butyrate and Propionate by the Rumen Epithelium

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Abstract
Epithelial tissue from the rumen of a mature steer was incubated with a trace of either n-butyrate-l-Cl4 or propionate-1-C14, plus a larger quantity of unlabeled n-butyrate, propionate or a 1:1 mixture of these acids. Only the C14-labeled substrates were added to the controls. After incubation, the samples were analyzed for butyrate, propionate, acetate, beta-hydroxybutyrate and lactate by partition chromatography. Each acid was subsequently analyzed for its content of C14. The C14 of labeled propionate was incorporated into lactate to an appreciable extent, but not into acetate, butyrate or beta-hydroxybutyrate. The C14 of labeled butyrate was incorporated into acetate and beta-hydroxybutyrate, particularly the latter acid, though not into propionate. In the presence of unlabeled propionate, appreciable C14 from labeled butyrate was incorporated also into lactate, though this did not occur in the absence of unlabeled propionate. These results are in accord with the concept that a portion of n-butyrate is metabolized by rumen epithelium via the citric acid cycle. These observations also indicate that the metabolism of propionate occurs through the fixation of CO2 to form succinyl Co A, the lactate presumably arising later through the decarboxylation of either malate or oxaloacetate. Whether the metabolism of propionate by rumen epithelium is a reflection of carbohydrate synthesis by this tissue remains to be established.