DNA Synthesis and Nucleoside Metabolism in Human Tonsillar Lymphocyte Subpopulations

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Abstract
High (HD) and low (LD) density cells were separated on 25% BSA gradient from tonsils of 3–6 years old children. Early B lymphocyte markers and slg-s were found on the surface of 59–82% of the LD cells. This cell population was 5–6 times more active in DNA synthesis (3H-thymidine incorporation, DNA polymerase activity) than the HD cells. The total uptake of 3H-deoxycytidine was about the same as that of 3H-thymidine. As long as practically all thymidine taken up by the cells was immediately incorporated into DNA (90–95%), only 10–15% of deoxycytidine was incorporated into DNA under the same conditions, indicating different pool sizes for the DNA precursors. The majority of deoxycytidine (70%) was converted and incorporated as dTMP. A considerable part of labeled deoxycytidine could be detected in the soluble pool in form of nucleotides (3–8%), and in an unknown form, called substance X (8–14%). Substance X was purified by TL chromatography and identified by HPLC as deoxycytidine containing liponucleotides, probable precursors for plasmamembranes. The preferential utilisation of deoxycytidine for DNA and membrane synthesis in immature B lymphocytes draws the attention to its function in early events of lymphocyte maturation.