The distribution of fixed carbon in amino acids.

Experiments previously reported from this laboratory (1) were designed to measure the normal amount of COz carbon which becomes incorporated into the amino acids of the tissues of rats fed a natural diet. This evaluation was made possible by exposing rats to CY402 for periods long enough to insure a complete exchange of isotope between CO2 and the organic constituents of several tissues. It was noteworthy that about one-fourth of the carbon of the carboxyl groups of liver aspartic and glutamic acids was derived from carbon dioxide. Smaller amounts of fixed carbon were found in the other non-essential amino acids, and no results were obtained which would suggest metabolic pathways of amino acid synthesis differing from those currently accepted. The present experiments were designed to measure the extent to which the non-essential amino acids of the diet affect the incorporation of COzderived carbon into tissue amino acids; this was done by offering the animals a diet free of these compounds during exposure to isotopic COz and comparing the results with those previously obtained. We might also expect to learn more about the synthetic pathways in the absence of dilution of synthetic amino acids by dietary amino acids and to measure the maximal extent to which CO2 may be used as a carbon source. The results obtained show that more CO2 becomes incorporated into the amino acids when they are absent from the diet, and that the increases differed from one metabolically related group of amino acids to another.