Metabolism of pyrrolizidine alkaloids in the ovine rumen. I. Formation of 7-α-hydroxy-1-α-methyl-8-α pyrrolizidine from heliotrine and lasiocarpine

Abstract

In vitro studies have demonstrated that the rate of metabolism of pyrrolizidine alkaloids in sheep's rumen fluid varies greatly. Fluids from sheep at pasture display much higher activity than do those from chaff-fed animals. Addition of dried Heliotropium europaeum to a chaff ration leads to a marked increase in the in vitro rate of alkaloid degradation by the rumen fluid but activity rapidly declines again when the H. europaeum is withdrawn. Thus a major factor limiting multiplication in the rumen of bacteria responsible for alkaloid breakdown appears to be lack of their specific substrates. N-oxides of pyrrolizidine alkaloids are rapidly reduced to tertiary bases in rumen fluid, even in samples from sheep not previously exposed to these substrates. The stimulation of the alkaloid-metabolizing system by vitamin B12 also varies greatly, both in whole rumen fluid and in fluids with activity reduced by removal of coarsely particulate matter. It is concluded that the B12 effect is not related to a specific requirement of the alkaloid-utilizing bacteria but to stimulation of other species which produce a metabolite essential for pyrrolizidine alkaloid metabolism.