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 <0.4 g/kg liveweight per day. Resorcinol glucuronide and the glucuronide of 2-carboxy-2'4'4,6,-tetrahydroxy diphenyl 2, 2'-lactone were the major urinary metabolites derived from tamic acid and probably from yellow-wood HT while resorcinol glucuronide was the major product of gallic acid metabolism. Minor urinary metabolites included unconjugated pyrogallol, resorcinol and phloroglucinol. Toxicity appeared to correlate with the passage of the diphenyl lactone metabolite, presumably arising from the degradation of the hexahydroxydiphenic acid moiety in HT. The current studies indicate that yellow-wood toxicity probably occurs under circumstances when animals ingest leaf containing high levels of HT without prior conditioning. A diagnosis of yellow-wood toxicity could be confirmed by the detection of resorcinol and diphenyl lactone metabolites in urine of affected ruminants.