Abstract
The tyrosine toxicity that occurs in rats fed a 10% casein diet containing 5% tyrosine can bealleviated by supplement of extra casein or methionine plus threonine.
In this study, the effects of addition of extra casein or methionine and threonine on the ability of tyrosine oxidation were measured using radioactive tyrosine. Weanling rats were fed ad libitum the 10% casein, 10% casein containing 0.66% L-methionine and 0.90% L-threonine, and 30% casein diets, with or without 5% L-tyrosine for 1 or 2 weeks. After a 12 hours fast, animals were ingested a difinite amount of each experimental diet, then injected intraperitoneally with 2μCi of L-tyrosine-U-14C, and expired CO2 was collected at hourly intervals for 10 hours. In all groups under investigation the rate of tyrosine oxidation was very rapid and reached a maximum in 1 or 2 hours after injection of radioactive tyrosine, then fell sharply to a much lower level. The catabolic rate of radioactive tyrosine to 14CO2 was high in rats fed the diets containing excess tyrosine, but not further increased by the addition of extra casein or methionine plus threonine under the experimental condition.
