The transfer of carbon from propionate to amino acids in intact cows.

Propionate, like acetate, is an active metabolite which arises from a variety of sources in animal tissues. It is produced during the metabolism of fatty acids which contain an odd number of carbon atoms (2) and which, as Shorland and Hansen (3) have shown, occur naturally in small amounts in triglycerides from various animal tissues. It arises also during the catabolism of certain amino acids including isoleucine (4), valine (5), methionine (6, 7), threonine (in part) (7, S), and norleucine (7). In ruminants, large quantities of acetate and propionate arise in the rumen and together constitute an appreciable part of the daily caloric intake for these animals (9, 10). The metabolic fate of these two fatty acids is different, propionate being largely glucogenic (11, 12) while acetate is largely lipogenic (13) or ketogenie (14). Thus, one might expect a difference in their utilization for amino acid synthesis. In an earlier study with acetate-CY4, it was shown that, among the amino acids, those derived directly from tricarboxylic acid cycle intermediates, glutamic and aspartic acids, had the greatest specific activities (15). The present study was undertaken to evaluate the role of propionate as a precursor of amino acids in casein. Five cows were injected intravenously with propionate-l-Cl4 or propionate-2-CY4, and amino acids were recovered from nineteen samples of casein collected at different times after injection. The results indicated that propionate was equally as important as a precursor for aspartic acid and serine as it was for carbohydrate. Surprisingly, alanine, a glucogenic amino acid, was formed with relatively small specific activity. These results are considered together with those previously obtained from cows injected with acetate-Cl4 (15) and glucose-Cl4 (16).