Studies in protein metabolism with compounds labeled with radioactive carbon; metabolism of dl-tyrosine in the normal and tumor-bearing rat.

In 1939 Schoenheimer, Ratner, and Rittenberg (1) employed nn-tyrosine labeled with heavy nitrogen (N15) to study the metabolism of this amino acid and the fate of its amino nitrogen in rats. The present investigation deals with the metabolism in rats of radioactive nn-tyrosine (tyrosine”), labeled in the /3 position with the long lived Cl* isotope of carbon (2).l The labeling of the carbon skeleton made it possible to measure the incorporation of tyrosine* into tissue proteins and the conversion of this amino acid into other compounds. The moderately high radioactivity of the tyrosine” permitted the administration of single small doses (5 mg.), well within the normal physiological range, and t,he subsequent measurement (with an accuracy of about 5 per cent) of fractions of the order of 0.001 to 0.0001 part of such doses. The present experiments revealed that the carbon chain of the tyrosine, unlike the amino nitrogen, does not contribute significantly to the formation of other amino acids.