Modulation of metabolism of cytosine arabinoside in L cells by antibody and complement

Abstract

Complement modulated the metabolic conversion of cytosine arabinoside into its nucleotide and nucleic acid derivatives in L cells by augmenting the biphasic effect (inhibitory/stimulatory) of anti-L cell antibody. Antibody in the absence of complement affected both the total cellular pool of cytosine arabinoside (containing the nucleotide and nucleic (DNA) acid derivatives) and the acid-insoluble pool (containing the DNA derivative). Addition of complement in high concentrations inhibited and, in low concentrations, stimulated the incorporation of cytosine arabinoside into DNA of antibody stimulated L cells. At the point of maximum stimulation of cytosine arabinoside incorporation into DNA, there was a pronounced decrease in the size of the total cellular pool with the result that 75% of the total pool of cytosine arabinoside was due to its incorporation into DNA. Simultaneous measurements upon cytosine arabinoside and thymidine or deoxycytidine metabolism were made; the effects of antibody and complement appeared to be quite similar with all three nucleosides, with the exception that all thymidine and deoxycytidine taken up into the cells was incorporated into DNA. Addition of exogenous thymidine (up to 100 μM produced the same enhanced incorporation of [3H]Ara-C and [14H]dCyd into control cells that was observed in antibody and complement stimulated cells. Measurement of deoxycytidine kinase activity in cell-free extracts of cell cultures stimulated by antibody, however, did not reveal enhanced enzymatic activity due to stimulated enzyme synthesis, but, rather indicated that antibody and complement altered the intracellular concentrations of nucleotides which affected the interaction of cytosine arabinoside with deoxycytidine kinase in intact cells. Indeed, significantly more mono (23 pmol/106 cells) and triphosphorylated (0.7 pmol/106 cells) nucleotide derivatives of cytosine arabinoside were produced in intact cells stimulated by antibody.