Metabolism of labeled aminoethanol, glycine, and arginine in the chick.

Wixom et al. (1) reported that supplementation of the diet with aminoethanol equivalent to 1.5% glycine increased the grouTth of chicks on a glycine-deficient 35% casein diet to the same extent as did supplementation with glycine. Such a magnitude of growth response implies an efficient conversion of aminoethanol to glycine. However, with the use of a 20 $& casein ration we were unable to obtain any growth response from aminoethanol, although a response was obtained from glycine. A possible explanation for these different results may be that the chicks on the higher protein diet required relatively more glycine for uric acid synthesis because of the greater protein catabolism, and that, under these conditions, aminoethanol also served as a uric acid precursor. To study this possibility further, experiments were performed with labeled glycine and aminoethanol and later with labeled arginine, since it is a precursor of part of the urinary creatine and creatinine molecules not derived from glycine. Investigations have established the following biosynthetic pathways in the rat: choline + betaine --+ dimethylglycine + sarcosine --+ glycine; aminoethanol -+ glycolaldehyde -+ glycolic acid + glyosylic acid + glycine; and triose -+ hydroxypyruvate --f serine (2-4). The formation of aminoethanol from glycine in vivo was shown to proceed via serine by decarboxylation (5), thus completing an aminoethanol-glycine cycle. However, the amount of glycine synthesized from choline in the rat appears to be quantitatively unimportant, and neither aminoethanol (6) nor dimethylaminoethanol (7) is converted readily into glycine. The pathway of purine biosynthesis has been thoroughly established, and the reactions involved in the incorporation of the 2 carbon atoms of glycine into the 4 and 5 positions of the purine molecule have been elucidated (8) with the use of pigeon or chick liver preparations. Extensive labeling of the glycine pool and of the 4 and 5 positions of excreted uric acid would thus be espected if labeled aminoethanol were converted to glycine.