The incorporation of formate-C14, glycine-2-C14, adenine-4, 6-C14, and phosphate-P32 into nucleic acids.

nine labeled either with C'4 or N'5. Considerable controversy has developed as to which one of these tracer compounds gives an accurate measurement of the “relativeturnover-rate― of the nucleic acids. In order to determine the “absolute turnover rate― of the nucleic acids, it is necessary to know the specific activity of the immediate precursor of the nucleic acids. Since even the identity of this immediate precursor is not known at present, it is necessary to find another way to compare the rate of incorporation of the following precursors—in organic phosphate (p32), formate-C'4, glycine-@-C'4, and adenine-4,6-C'4—into the nucleic acids. Judg ing from the varied metabolic paths of these label ing agents, their “absolute availability― to the nucleic acids is probably not the same. Assuming that the labeling agent within any one tissue is equally available to both PNA and DNA, it seems advisable to compare the specific activity (per cent incorporation of radioactivity/mg nucleic acid phosphorus) of PNA to that of DNA, with the use of a common precursor. If this specific activity ratio is constant irrespective of the pre cursor used, then the molecules of PNA and DNA are renewed in toto, or at least the labeled part of the molecule is renewed at an equal relative rate in PNA and DNA. In several earlier papers the PNA :DNA specific activity ratios were compared. These ratios as re ported in the literature vary considerably, not only for the different precursors employed but also from author to author utilizing the same precursor. The findings for the PNA :DNA ratios in rat livers are tabulated in Table 1.