Origin of pyrimidine deoxyribonucleotide pools in perfused rat heart: implications for 3'-azido-3'-deoxythymidine-dependent cardiotoxicity.

In adult non-replicating tissues such as heart, demand for dNTPs (deoxynucleoside triphosphates) is low but essential for mitochondrial DNA replication and nuclear DNA repair. dNTPs may be synthesized from salvage of deoxyribonucleosides or by reduction of ribonucleotides. We have hypothesized that the cardiac mitochondrial toxicity of the nucleoside analogue AZT (3'-azido-3'-deoxythymidine; known as zidovudine) is caused by inhibition of thymidine kinase 2 of the salvage pathway and subsequent TTP pool depletion. The extent to which this hypothesis has merit depends on how much the heart relies on thymidine phosphorylation for maintenance of the TTP pool. In the present study, we used isotopic tracing to demonstrate that both TTP and dCTP are solely synthesized by phosphorylation of thymidine and deoxycytidine respectively, with no evidence for synthesis from other precursors. We have also shown that UTP and CTP are synthesized by phosphorylation of uridine and cytidine respectively, with no detectable role for the de novo pyrimidine synthesis pathway. Lastly, we have demonstrated that AZT decreased the TTP pool by 50% in 30 min of perfusion, while having no effect on other dNTPs. In summary, the present study demonstrated that adult rat heart has a limited mechanism for dCTP and TTP synthesis and thus these pools may be more sensitive than replicating cells to drugs such as AZT that affect the salvage pathway.