Introduction: Protein or methionine restriction in the diet is known to decrease reactive oxygen species (ROS) production and mitochondrial oxidative stress and to increase maximum longevity in rodents, which could explain how these changes also take place in dietary restriction. However, it is not known whether restriction of other amino acids is also involved. To clarify this question, we studied the effect of restricting all the amino acids, except methionine, of the semi-purified diet, AIN 93G, in Wistar rats.
Material and methods: Seven-week old male Wistar rats (n=16) were randomly divided into two groups: a control group and a group with 40% restriction of dietary amino acids except methionine. After 7 weeks of dietary treatment, the animals were sacrificed and their livers were extracted to isolate mitochondria immediately and measure ROS production and oxygen consumption; these data allowed the percentage of free radical leak to be calculated. Oxidative damage to mitochondrial DNA was calculated as 8-oxo-7,8-dihydro-2'-deoxyguanosine by HPLC-EC.
Results: At the end of the experimental period, a decrease in kidney weight was observed, but the weight of the liver, heart and brain was unchanged. ROS production in isolated liver mitochondria was unchanged with complex I (pyruvate/malate or glutamate/malate) or complex II (succinate) linked substrates. Maximum rates of ROS production significantly decreased with glutamate/malate+rotenone but not with pyruvate/malate+rotenone or with succinate. There were no changes in oxygen consumption with any substrate either in state 4 (resting) or in state 3 (phosphorylating). In agreement with the ROS production results, there were no differences between groups in oxidative damage to mitochondrial DNA.
Conclusions: Taken together with previous results concerning methionine restriction, the results obtained in the present study clearly show that the decrease in ingestion of only one molecule, methionine, causes the decrease in ROS production and oxidative damage to mitochondrial DNA that is observed in dietary restriction in relation to the decrease in the rate of aging.