Hydrolysis products of nucleic acids labeled with tritium; preparation by biosynthesis.

Compounds labeled with Cl4 and N16 have proved useful in studies of nucleic acid metabolism. Since tritium, the radioactive hydrogen isotope of atomic weight 3, is now available in relatively high specific activities, it was decided to explore the usefulness of this isotope in such studies. Inasmuch as there are no data concerning the corresponding use of deuterium, it was first considered necessary to know the relative stability of the hydrogen atoms bonded to carbon with respect to isotopic exchange with an aqueous medium at varied pH and temperature. It is known that. hydrogen atoms are labile when bonded to oxygen and nitrogen in --OH, -NH*, and =NH, and to carbon at a site where enolization can occur (1). The ribose port,ion of the nucleosides and nucleotides would be expected to retain an isotopic label under moderate reaction conditions, since 6 hydrogen atoms are linked directly to carbon. A measure of stability would be predicted for the hydrogen atoms at. the 2 and 8 positions in adenine, the 8 position in guanine, and the 4 and 5 positions in cytosine and uracil, provided that the purine and pyrimidine ring st,ructures did not introduce any unusual lability fact,ors. In order to resolve t,his point, the four bases were prepared by degradation of sodium nucleate from yeast labeled wit,h tritium. The bases retained appreciable tritium activity after undergoing treatment that would result in isotopic loss at labile atom positions. The corresponding nucleosides were prepared from the separated nucleotides and their tritium activities measured. The biosynthetic method used in these studies effected a multifold growth of yeast in a nutrient medium that forced the incorporation of appreciable tritium activity. The method is equally suitable for the incorporation of radiocarbon activity, thus permit,ting the biosynt,hesis of