Utilization of massive doses of amino acids in protein and porphyrin biosynthesis

¹ Radioisotope Service, Veterans Administration Hospital, Brooklyn, New York

Rats were injected intravenously with doses up to 250 mg of glycine containing a tracer quantity of glycine-2-C¹⁴. Between 1 and 15 days the animals were exsanguinated and globin, hemin, and plasma proteins isolated and counted. The relative specific activity (i.e., counts per minute per 100 mg sample/counts per minute injected per gram body wt.) found in globin, hemin, and plasma proteins was only slightly affected by the size of the administered dose of glycine. Thus, typically, the relative specific activity of the globin of rats injected with 0.015 mg was only two to three times greater than that in rats injected with 250 mg; for the hemin this difference was somewhat larger but still less than fourfold. Determination of relative specific activity values for globin and plasma proteins when small or large doses of methionine were injected, yielded results similar to those found with glycine. The total per cent of excretion, in the urine and in the expired air, of the radioactivity administered with the different doses of glycine, was also found to be quite similar. This indicates that with increasing injected doses the rat is able to retain and utilize larger quantities of the amino acid. Possible explanations for these results are enumerated and the significance of these findings in relation to some previous studies is indicated.