Biosynthesis of Amino Acids by Ruminal Microorganisms

Cellular protein in ruminal microorganisms generally constitutes a large and important part of the a-amino nitrogen assimilated by the host. In sheep fed wheaten hay, 63 to 82% of ruminal nitrogen was in microbial cells, with the proportion in cells increasing with time after feeding (Weller, Gray and Pilgrim, 1958). The concentration of extra-cellular amino acids in the rumen is usually relatively low (Wright and Hungate, 1967a). Free amino acids are rapidly catabolized and velocity constants (K values) of 0.81 to 1.4/ min. were measured for glycine (Wright and Hungate, 1967b); 0.76/min. for aspartic acid and 0.58/min. for glutamic acid (Portugal, 1963). Since only a small proportion of these amino acids are incorporated intact into microbial proteins, de novo synthesis of amino acids by ruminal microbes is an important activity, even in animals fed natural rations. The importance of these biosynthetic reactions is most evident when ruminants grow and produce milk protein when they are fed diets containing few or no amino acids (Loosli et al., 1949; Virtanen, 1966),since those amino acids essential for rats are not synthesized by ruminant tissue enzymes (Black and Kleiber, 1962).