Permeation and salvage of dideoxyadenosine in mammalian cells.

Transmembrane equilibration of 2',3'-dideoxyadenosine (ddAdo) was measured by rapid kinetic techniques in deoxycoformycintreated P388 and L1210 mouse leukemia cells and human erythrocytes, at 25 degrees. It was only about 10% as rapid as that of other purine nucleosides that are known substrates for the nucleoside transporters of these cells. ddAdo entry was nonsaturable up to a concentration of 1 mM and was not inhibited by other nucleosides or two nucleoside transport inhibitors, dipyridamole and nitrobenzylthioinosine. Thus, ddAdo permeation was mainly nonmediated. It was relatively rapid because of the high lipid solubility of ddAdo. ddAdo entered the cells at least 100 times more rapidly than dideoxycytidine but less rapidly than trideoxythymidine, with an even greater lipophilicity than ddAdo. ddAdo was not phosphorylated in human erythrocytes, but there was some phosphorylation in deoxycoformycin-treated P388 and L1210 cells. In situ conversion of 10 microM ddAdo to ddATP, however, was slow and ceased after 5-10 min at 25 degrees or 37 degrees. Cessation of net uptake was not due to turnover of dideoxy-ATP or deamination of dideoxy-AMP. The results suggest that ddAdo salvage in the absence of deamination is limited by feedback inhibition of its phosphorylation, perhaps by deoxycytidine kinase. Permeation into the cells was not rate limiting to ddAdo salvage. In P388 and L1210 cells that had not been treated with deoxycoformycin, ddAdo was salvaged at least 100 times more efficiently than in deoxycoformycin-treated cells and converted to nucleoside triphosphates, but the end-products and pathways of salvage have not been resolved entirely. Salvage of ddAdo required deamination but was not primarily via dideoxyinosine----hypoxanthine----IMP, as is the case for 2'-deoxyadenosine salvage, because [3H]ddAdo salvage was only little inhibited by unlabeled hypoxanthine, whereas it was strongly inhibited by 2'-deoxyadenosine, adenosine, and adenine.