The biosynthesis of proteins. 2. Synthesis of milk proteins by the goat

Abstract

A lactating goat was injected with radioactive glycine, valine, lysine and methionine, and the appearance of radioactivity in the milk proteins was followed. After injection of [S35] methionine, both casein and whey proteins reached maximum activity in about 1.5 hour. The whey activity curve followed closely the casein curve but was always slightly below it. After simultaneous injection of [2-C14] glycine, DL-[4-C14] valine and DL-[1-C14] lysine, milk whey was fractionated and a crystalline beta-lactoglobulin was isolated. This protein and a sample of casein from the same milk were hydrolyzed and the radio-active dinitrophenyl amino isolated in a pure state. There was no significant difference in radioactivity between glycine, valine and lysine from casein and the corresponding amino acids from lactoglobulin. Casein serine was more active than lactoglobulin serine. The significance of these results is discussed.