Conversion of Methionine to Cystine: Experiments with Radioactive Sulfur (S35)

Abstract

A considerable amount of evidence pointing to the idea that methionine, when fed, may be converted into cystine in the animal organism has been brought forth. Further evidence in support of this concept is afforded by the present experiments in which it is shown that cystine containing the radioactive sulfur isotope (S35) was isolated from rats that were fed methionine containing S35. The methionine was synthesized from sulfur that contained S85 by a modification of the procedure described by Patterson and du Vigneaud. It was administered to young rats that were maintained on a low cystine diet. Animals 20 and 24 received 0.15% of the S35-containing methionine daily in their diet, while rat 22 was given daily 15 mg of the same methionine subcutaneously. Rats 32, 34, and 37, maintained on the same basic diet, were fed 0.15% of ordinary methionine daily plus 0.10% of sodium sulfate that contained radioactive sulfur. The radioactivity of the sodium sulfate was about 2/3 of that of the methionine. After a period of 5 weeks, cystine was isolated from the hair, skin, or the whole carcass of the animals by hydrolysis of the protein, precipitation of the cystine as the cuprous mercaptide, and purification by reprecipitation. The amino acid was finally obtained in pure crystalline form. It was then converted into barium sulfate. The radioactivities of the barium sulfate samples were kindly measured by Dr. W. F. Libbey with the aid of the screen wall Geiger counter. The cystine isolated from the hair of rats 20 and 22, the skin of rat 20, and the whole carcass of rat 24 contained radioactive sulfur. The radioactivity of the cystine isolated from the whole carcasses of rats 32, 34, and 37 was zero within the error of measurement.