To investigate the role of transmethylation metabolites in initiation of apoptosis in human leukemia HL-60 cells exposed to 3-deazaadenosine (c3 Ado) as single agent and c3Ado plus homocysteine thiolactone (Hcy), S-adenosylmethionine (AdoMet), S-adenosylhomocysteine (AdoHcy) and 3-deazaadenosylhomocysteine (c3AdoHcy), were measured by HPLC in HL-60 cells exposed to 1-100 microM c3Ado as single agent and 1 to 100 microM c3Ado plus 1 mM Hcy. 3-deaza-(+/-)-aristeromycin (c3Ari), a more specific and potent S-adenosylhomocysteine hydrolase inhibitor compared with c3Ado, was used to inhibit synthesis of c3AdoHcy. AdoMet increased to maximum values 1.5- and 2.3-fold control in cultures treated with c3Ado as single agent and c3Ado plus Hcy, respectively. AdoHcy did not change in cultures treated with c3Ado as single agent, but increased to maximum values 2.0-fold control when Hcy was added. The synthesis of c3AdoHcy was favored by Hcy, and 35- to 70-fold higher levels of c3AdoHcy were found in cultures treated with c3Ado plus Hcy vs. c3Ado as single agent. The amounts of c3AdoHcy in cultures treated with c3Ado at concentrations initiating apoptosis did not exceed c3AdoHcy levels in cultures treated with c3Ado plus Hcy at concentrations not initiating apoptosis. Pretreatment of c3Ado plus Hcy cultures with c3Ari diminished c3AdoHcy and almost completely abrogated apoptosis. Exposure of cells to 100 microM c3Ari as single agent resulted in an increase in AdoHcy to 8.4-fold control but no changes in AdoMet and no initiation of apoptosis. Our findings indicate that c3Ado as single agent exerts its apoptosis-initiating effect through a nontransmethylase-related bio-chemical action. c3AdoHcy seems to be related to biochemical events initiating apoptotic cell death of HL-60 cells when c3Ado and Hcy are combined.