Conversion of 5′-Deoxy-5′-methylthioadenosine and 5′-Deoxy-5′-methylthioinosine to Methionine in Cultured Human Leukemic Cells¹

5′-Deoxy-5′-methylthioadenosine and 5′-deoxy-5′-methylthioinosine, which are metabolized to the methionine precursor, 5-methylthioribose-1-phosphate, by 5′-deoxy-5′-methylthioadenosine phosphorylase and purine nucleoside phosphorylase, respectively, can serve as sources of methionine for cultured HL-60 promyelocytic leukemia cells. CCRF-CEM T-cell leukemia cells, which lack 5′-deoxy-5′-methylthioadenosine phosphorylase, convert 5′-deoxy-5′-methylthioinosine (but not 5′-deoxy-5′-methylthioadenosine) to methionine; this conversion is blocked by purine nucleoside phosphorylase inhibitors. Therefore, the pathway for the conversion of 5-methylthioribose-1-phosphate to methionine is present in both human leukemic lines.