Metabolism of DL-methionine and methionine analogs by rumen microorganisms

Rates of degradation of DL-methionine and a number of methionine derivatives by rumen microorganisms were studied in vitro. Methionine hydroxy analog, the ammonium salt, and the amide derivative of methionine hydroxy analog were degraded more slowly than was methionine. Methyl and ethyl esters of methionine hydroxy analog were rapidly converted to methionine hydroxy analog, which was then degraded. Whole rumen contents were separated into protozoal and bacterial fractions, and rates of disappearance of [14C]carboxyl-labeled methionine and methionine hydroxy analog were determined. Disappearance of the label tended to be slower in the bacterial fraction; however, incorporation into cellular material tended to be higher for the bacterial than for the protozoal fraction. Disappearance of labeled methionine hydroxy analog was slower than labeled methionine in all fractions. Addition of unlabeled methionine inhibited disappearance of labeled methionine hydroxy analog, but unlabeled methionine hydroxy analog did not affect disappearance of labeled methionine. The effect of either Na2SO4, methionine, or methionine hydroxy analog on neutral detergent fiber digestion was related to amount of sulfur in the medium and not source of sulfur.