Metabolic Fate of Cysteine and Methionine in Rumen Digesta

Abstract

Estimates were obtained of the extent to which cysteine and methionine were incorporated into the protein of the microbes of rumen digesta without prior degradation and resynthesis. By using the amino acids labeled with both 35 S and 14 C, it was observed that a large proportion of the 35 S appeared in the sulfide pool and of the 14 C appeared in volatile fatty acids. By isolating the appropriate amino acid, obtaining the 14 C to 35 S ratio, and comparing this with the ratio in the added amino acid, the degree of direct incorporation was calculated. For cysteine it was estimated that at most 1% and for methionine, at most 11% of the amino acid in the free pool was incorporated unchanged into microbial protein. As a consequence of these findings, it is considered that the method for measuring microbial protein synthesis in rumen digesta based upon incorporation of 35 S from the free sulfide pool is not seriously affected by direct utilization of sulfur amino acids arising from dietary sources.