Turnover of nucleic acids in a non-multiplying animal cell

Abstract

Turnover of ribonucleic acid and deoxyribonucleic acid was studied in rabbit macrophages. These cells remain alive and motile in vitro for long periods but do not multiply. The amounts of ribonucleic acid and protein per cell remain constant. No synthesis of deoxyribonucleic acid and no turnover of deoxyribonucleic acid adenine or thymidine residues could be detected. As measured by the incorporation and release of [8-C14] adenine, a rapid turnover of ribonucleic acid occurred. It could be shown, however, that the breakdown products of this turnover were reincorporated into ribonucleic acid with great efficiency. The observed rate of turnover was thus determined by the ease with which an unlabelled exogenous compound could enter the pathway by which the products of turnover were recycled and do displace or dilute a labelled intermediate. In this respect adenine, the 2[image]-, 3[image]- and 5[image]-phosfate of adenosine and inosine were all less effective than adenosine. In the presence of exogenous adenosine the observed half-life of the ribonucleic acid involved in turnover was less than 2 hr; with the other compounds mentioned the observed half-life was about 12 hr. With inosine 5''-phosfate no turnover could be detected. As labelled adenine was displaced from the ribonucleic acid, a comparable amount of labelled inosine appeared in the culture medium. P32 was incorporated into ribonucleic acid but could not be displaced from it by unlabelled orthophosphate over the period studied.