RESUMEN
Gamma-glutamyltranspeptidase (GGT, EC 2.3.2.2) activity was determined in the plasma and liver of 40 young (50 days old) and 40 mature (300 days old) male Wistar rats, after a protein restriction period of 28 days. Casein protein levels used were: 1 percent, 3 percent, 5 percent, 7 percent and 28 percent (control). Weanling rats submitted to protein-free diet and adult rats submitted to that and other low-protein diets (1 percent, 3 percent or 5 percent casein) presented weight reductions (-0.38 +/- 0.07 g/day and -0.98 +/- 0.19 g/day, respectively). Only in young animals did these weight reductions parallel those of food consumption (37 +/- 12 percent of control), plasma protein (52 +/- 11 percent of control), plasma albumin (70 +/- 12 percent of control), hepatic RNA (68 +/- 7 percent of control) and protein (71 +/- 8 percent of control). A marked effect of the protein restriction, increasing the GGT activity, was also observed only in young rats. A significant (P<0.05) rise was promoted by the protein-free diet in the plasma GGT (2.83 +/- 1.39 vs 0.69 +/- 0.50 mU/ml for control) and by both the protein-free and 3 percent casein diets, in the liver GGT (respectively, 16.00 +/- 6.72 and 7.75 +/- 3.49 vs 0.94 +/- 0.57 U/g protein for control). The different results obtained for young in relation to mature animals could be explained by the reduction of both protein and sulfur-containing amino acid requirements with aging.