Effect of dietary level of protein or methionine and threonine on the amino acids and catecholamines in brain of rats fed a high tyrosine diet.

When rats were fed a high tyrosine diet (5% in diet), plasma tyrosine, brain tyrosine, norepinephrine (NE), and 3-methoxy-4-hydroxyphenylethyleneglycol-sulfate (MHPG-SO4) were increased. The concentrations of brain tyrosine were directly correlated with those of plasma tyrosine and tyrosine ratios (tyrosine/large neutral amino acid (LNAA)) in plasma and brain. When rats were fed a high tyrosine diet with increasing amounts of the dietary protein, the contents of brain tyrosine and MHPG-SO4 decreased stepwise. By the supplementation of both methionine and threonine to the high tyrosine diet, the enhancement of brain tyrosine and MHPG-SO4 caused by tyrosine was significantly reduced. These results suggest that effects of tyrosine toxicity such as growth retardation or external pathological lesions may be due to the accelerated NE-turnover in the brain accompanied by the enhancement of brain tyrosine concentration. The supplemented methionine and threonine may cause an increase of tyrosine utilization, because methionine and threonine are the most limiting amino acids in casein, and also may inhibit the uptake of tyrosine into the brain via the blood-brain barrier.