Aspects of Threonine and Glycine Metabolism in the Chick (Gallus domesticus)

Two assays were conducted with young chicks to assess the nutritional implications of the degradation of excess threonine to glycine and acetaldehyde. In the first experiment, 1.0% threonine added to a semi-purified diet depressed growth and food intake severely, and also lowered the circulating levels of glycine. In the second assay, 0.5% threonine added to a crystalline amino acid diet lacking in glycine and serine but, containing 0.3% threonine, enhanced weight gain and efficiency of food conversion but had no appreciable influence on plasma glycine levels. Addition of 1.0% threonine to the crystalline amino acid diet did not augment growth further. Combinations of 1.2% glycine and threonine supplements increased growth markedly, 0.5% additional threonine producing the best performance. The plasma amino acid data, therefore, suggest that threonine is not readily degraded to glycine and, therefore, cannot act as a precursor of this amino acid for the chick.