Replacement of L-phenylalanine and L-leucine by alpha-hydroxy analogues in the diets of germfree rats.

It has been suggested that the metabolic activity of the intestinal microflora is necessary for the utilization of the alpha-keto and alpha-hydroxy analogues of the essential amino acids. This role of the intestinal microflora has been examined by comparing weight gain of germfree and conventional rats in response to the substitution of one amino acid in the diet by its analogue. Using a completely defined liquid diet it was found that weight gain of germfree rats was well maintained and comparable to that of conventional rats when 1.5 M equivalents of DL-alpha-hydroxyisocaproic acid was substituted for L-leucine in the diet. Although weight gain was not fully maintained when 2.0 M equivalents of L-phenyllactate was substituted for L-phenylalanine, germfree and conventional rats showed similar weight gains. These results exclude an obligatory role for the flora in the utilization of these non-nitrogen containing analogues of L-phenylalanine and L-leucine.