RESUMEN
This investigation was conducted to clarify the role of cysteine on its methionine-sparing effect by studying in vivo the various parameters that are directly related to the metabolism of the methionine carbon, such as conversion to CO2, and incorporation of the methionine carbon into proteins, phospholipids and nucleic acids. At low (0.4%) dietary level of methionine, cysteine had a depressive effect on the oxidation of the methionine carbon to carbon dioxide, and with 0.51% cysteine in the diet the oxidation of the methyl or carboxyl carbon of methionine to CO2 was depressed by 29 and 20%, respectively. The amount of the label in urine samples was unaltered by the level of cysteine in the diet, and the body retention of either the methyl or carboxyl carbon of methionine was greater at low methionine intake when cysteine in the diet was increased. Under these conditions, rat growth was enhanced by feeding increasing amounts of cysteine. The incorporation of methionine methyl carbon into tissue nucleic acids and phospholipids was depressed by high (0.51%) levels of cysteine in the diet, at a low (0.4%) methionine intake. Under these conditions there was an increased incorporation of the methionine methyl or carboxyl carbon into tissue proteins. In addition, it appears that the resynthesis of methionine from homocysteine is enhanced by increasing the amounts of cysteine in the diet. All of these findings suggest that the addition of cysteine increases the avilability of methionine for protein synthesis and, hence, growth.