The metabolic stability of the nucleic acids in cultures of a pure strain of mammalian cells

Abstract

The pattern of incorporation of C formate into the nucleic acids of strain L mouse fibroblasts was studied. The metabolic stability of the nucleic acids was also investigated by measuring the loss of C14 from deoxyribonucleic acid, nuclear ribonucleic acid and cytoplasmic ribonucleic acid of labelled cells. The cells are capable of synthesizing purines and thymine, but preferentially utilize adenosine, guanosine and thymidine when these are present in the medium. Adenosine is more readily incorporated than guanosine and conversion of adenine into guanine takes place more readily than does the reverse reaction. Like some other tissues which have been studied in vitro, the cultured cells show a higher degree of incorporation of C formate into deoxyribonucleic acid thymine than into deoxyribonucleic acid purines when the medium contains plentiful amounts of purine and pyrimidine precursors. Incorporation into purines of both deoxyribonucleic acid and ribonucleic acid can be enhanced by addition to the medium of a mouse-liver extract or of 5-amino-4-imldazole carboxamide riboside. In the absence of preformed purines and pyrmidines in the medium twice as much (C14) formate is incorporated into the deoxyribonucleic acid purines as into thymine. When labelled cells are allowed to grow slowly in non-radioactive medium, activity is progressively lost from ribonucleic acid (in excess of the loss due to dilution). Cytoplasmic ribonucleic acid and nuclear ribonucleic acid behave similarly and some degree of turnover in both is suggested. Little loss of C14 from deoxyribonucleic acid is observed in similar conditions. However, when a large amount of thymidine is added to the medium the loss of activity from deoxyribonucleic acid thymine exceeds that found in control cultures, whereas the activities of adenine and guanine are unaffected. These observations suggest that there may be some turnover of deoxyribonucleic acid also but that this is obscured by reincorporation of breakdown products.