Metabolism in sheep of gallic acid, tannic acid and hydrolysable tannin from Terminalia oblongata

Abstract

Hydrolysable tannin (HT) is present in a variety of tropical browse plants, some of which poison ruminants. In an attempt to clarify the toxic action, we investigated the major urinary metabolites resulting from dosing of sheep with the HT, tannic acid; its simplest and major phenolic component, gallic acid; and the HT-containing and toxic Terminalia oblongata. Phenolic metabolites were separated by HPLC and their structures investigated by proton and 13C NMR. Gallic acid was less toxic (w/w) than tannic acid. Comparison of urinary metabolites from rumen and abomasal administration indicated that decarboxylation and reductive dehydroxylation of phenolics occurred principally in the rumen and a significant proportion was totally degraded. Rumen metabolism appeared to prevent toxicity from gallic and tannic acid at dose rates of