Effects of dietary sulfur-containing amino acids on oxidative damage in rat liver caused by N-nitrosodimethylamine administration

Effects of dietary protein and S-containing amino acids on oxidative damage were investigated in rat liver. After feeding rats for 3 weeks from weaning, lower GSH levels and higher metallothionein (MT) levels were found in the liver of rats fed on a 10% soyabean-protein-isolate (SPI)-based diet than those fed on a 10% casein-based diet. After injection of N-nitrosodimethylamine (NDMA) at 20 mg/kg body weight, increases in lipid peroxide, determined as thiobarbituric-acid reactive substances (TBARS), and gamma-glutamyltransferase (GGT) activity in the liver were observed in the 10% SPI diet group. By supplementing the 10% SPI diet with 0.3% cystine or methionine, GSH levels were increased, while MT levels were decreased, and elevation in TBARS levels after NDMA injection was diminished. On the other hand, increase in GGT activity could be prevented only by methionine supplement. On a 20% SPI or casein diet, TBARS concentration and GGT activity were not altered after NDMA injection with concomitant increase in GSH levels and decrease in MT levels. These results indicate that sufficient amounts of methionine and cystine in a diet are important to protect the liver from oxidative damage after NDMA administration, and GSH plays a primary role in the cellular protective function when compared with MT.