STUDIES ON THE METABOLIC FUNCTION OF BRANCHED-CHAIN VOLATILE FATTY ACIDS, GROWTH FACTORS FOR RUMINOCOCCI I

Abstract

Allison, Milton J. (Dairy Cattle Research Branch, U. S. Department of Agriculture, Beltsville, Md.), M. P. Bryant, and R. N. Doetsch. Studies on the metabolic function of branched-chain volatile fatty acids, growth factors for ruminococci. I. Incorporation of isovalerate into leucine. J. Bacteriol. 83:523–532. 1962.—Ruminococcus flavefaciens strain C94, a cellulolytic rumen bacterium, requires either isobutyrate or isovalerate for growth. The organism was grown in the presence of C¹⁴-labeled isovalerate, and the metabolic fate of the labeled carbon was studied to obtain information on the functions of this growth factor. Radioactivity from isovalerate-1-C¹⁴ and isovalerate-3-C¹⁴ was found mainly in the protein and lipid fractions of the cells. The C¹⁴ in protein was all in leucine, indicating that a function of isovalerate was to serve as a carbon skeleton for leucine synthesis. As C¹⁴ in leucine synthesized from isovalerate-1-C¹⁴ was entirely in carbon 2, the intact isovalerate molecule was apparently incorporated into leucine. This is evidence that leucine was synthesized by a mechanism different from that previously demonstrated in other microorganisms. R. flavefaciens has a definite but limited ability to incorporate exogenous amino acids, including leucine. It incorporated 2% of the C¹⁴ during growth in uniformly labeled (UL) C¹⁴-Chlorella protein hydrolyzate; Escherichia coli incorporated 37% of the label under similar conditions. In another experiment, a limited amount of exogenous leucine-2-C¹⁴ was incorporated into protein of R. flavefaciens. The requirement for isovalerate was not replaced by dl-leucine or 2-ketoisocaproate. It is suggested that isovalerate or isobutyrate is required because R. flavefaciens has a limited ability to incorporate exogenous branched-chain amino acids and a limited ability to synthesize the isopropyl group found in these amino acids and in other components of the cell. Images