Metabolism of acetate, propionate and butyrate by sheep-liver slices.

Short-chain fatty acids constitute a major energy source in ruminants (Annison & Lewis, 1959). These products of ruminal fermentation are absorbed directly into portal blood and pass to the liver before reaching the general circulation. Examination of arterial and portal blood in sheep has indicated that propionate and butyrate are almost completely removed by the liver under normal feeding conditions, but substantial quantities of acetate appear in peripheral blood (Annison, Hill & Lewis, 1957). A substantial glucose turnover in sheep has been indicated (Annison & White, 1961; Kronfeld & Simesen, 1961), focusing attention on the nature of glucose precursors in this species since very little glucose is absorbed from the alimentary tract. Propionate is well recognized as a source of glucose, but the possibility of butyrate and, to a less extent, of acetate being a precursor of glucose has been the subject of much speculation. In the present investigation no evidence was obtained of net synthesis of carbohydrate from butyrate or acetate in liver slices. The possible synthesis of glycogen from glucose or propionate in liver slices was studied in media containing several combinations of inorganic ions. Glycogen loss occurred in all media examined, although in a medium with high potassium content reported to favour glycogen synthesis in rat-liver slices (Hastings, Teng, Nesbett & Sinex, 1952) glycogen loss was minimal. The metabolic interrelations of acetate, propionate and butyrate in liver slices were also examined with 14C-labelled substrates. Incorporation of substrate into glucose, oxidation to [14C]carbon dioxide and production oflactate and ketone bodies were examined. Considerable ketone-body production from butyrate was observed, and the labelling pattern in P-hydroxybutyrate derived from [1-14C]butyrate was similar to that reported by Hird & Symons (1959) with rumen or omasum epithelium and the same substrate.