Effect of adding dietary methionine to a low soy protein diet on the brain protein synthesis rate in ovariectomized female rats.

A deficiency of sex hormones affects brain function in mammals, including the decrease of protein synthesis. Recently, we have shown that the protein synthesis in the brain depended on the quality of dietary protein in ovariectomized female rats. The methionine is the first limiting amino acid for the recommended dietary allowance of amino acids in soy protein. The purpose of this study was to determine whether the addition of dietary methionine affected the rate of brain protein synthesis in ovariectomized female rats fed on the soy protein diet. Experiments were conducted on two groups of ovariectomized female rats (24 week) given the diets containing 5% soy protein or 5% soy protein + 0.2% methionine for 10 d. The fractional rates of protein synthesis in cerebral cortex and cerebellum significantly increased with an addition of dietary methionine. In the brain, the RNA activity [g protein synthesized/((g RNA) x d)] was significantly correlated with the fractional rate of protein synthesis. The RNA concentration (mg of RNA/g of protein) was not related to the fractional rate of protein synthesis in any organ. The results suggest that the addition of limiting amino acid for the low soy protein elevates the rate of protein synthesis in the brain of ovariectomized female rats, and that RNA activity is at least partly related to the fractional rate of brain protein synthesis.

Dietary genistein affects brain protein synthesis rates in ovariectomized female rats.

The purpose of this study was to determine whether genistein affects the rate of brain protein synthesis in ovariectomized female rats. Experiments were conducted on three groups of 12-wk-old female rats: those in group 1 were ovariectomized to reduce the level of plasma sex hormone; those in group 2 were ovariectomized and fed diets containing 0.01% genistein; and those in group 3 were sham-operated controls. The fractional rates of protein synthesis in the brain of ovariectomized rats fed genistein were significantly greater than those in ovariectomized rats without genistein treatment. In the cerebral cortex and cerebellum, the RNA activity [g protein synthesized/(g RNA.d)] significantly correlated (r > 0.86, P < 0.001) with the fractional rate of protein synthesis. The RNA concentration (mg RNA/g protein) was not related to the fractional rate of protein synthesis in any organ. The results suggest that the addition of genistein to the diet of ovariectomized female rats is likely to increase the rate of protein synthesis in the brain, and that RNA activity is at least partly related to the fractional rate of brain protein synthesis.