Ruminal metabolism of plant toxins with emphasis on indolic compounds

Ruminal bacteria can perform biochemical transformations on plant constituents that may affect the health of ruminant animals. Reactions carried out by ruminal bacteria on oxalates and some pyrrolizidine alkaloids include decarboxylation, hydrolysis and reduction steps. Prior exposure of ruminal bacteria to these substances increases the rate of detoxification, indicating an adaptive response by the bacteria to these substrates. The formation of toxic substances by ruminal bacteria also occurs and may involve similar reactions. Hydrolysis of cyanogenic glycosides and miserotoxins , reduction of nitrate and S-methylcysteine sulfoxide to nitrite and dimethyl disulfide can result in toxicity in ruminants. Similarly, the deamination and decarboxylation reactions associated with the degradation of tryptophan and tryosine result in the formation of 3-methylindole and p-cresol, which are toxic. Formation of 3-methylindole results from fermentation of tryptophan to indoleacetic acid, with subsequent decarboxylation of indoleacetic acid to 3-methylindole by a Lactobacillus sp. The 3-methylindole causes acute pulmonary edema and emphysema in ruminants as a result of mixed function oxidase metabolism in tissues. The 3-methylindole is also the cause of naturally-occurring acute bovine pulmonary edema and emphysema after abrupt pasture change. Inhibition of ruminal 3-methylindole formation by monensin and other antibiotics lowers ruminal 3-methylindole concentrations and prevents acute lung injury in experimental animals.