Further elucidation of mechanism of resistance to vincristine in myeloid cells: role of hypochlorous acid in degradation of vincristine by myeloperoxidase

Inherent resistance of myeloblasts to vincristine (VCR) has been related to the activity of myeloperoxidase (MPO) which can degrade VCR in the presenc e of hydrogen peroxide (H2O2). We investigated the relationship between VCR degradation and hypochlorous acid (HOCI) generation from the reaction of H 2O2 with chlorine (CI) as catalyzed by MPG. A cell-free system, three human leukemia cell lines (CEM/CCRF, HL-60, U937) and 15 bone marrow samples fro m children with acute myeloid leukemia (AML) were studied. VCR cytotoxicity was evaluated by MTT assay and by quantitative measurement of apoptosis. I n vitro levels of VCR in cell-free systems were measured by high performanc e liquid chromatography (HPLC), and intracellular HOCI levels by oxidation of 5-thio-2-nitrobenzoic acid with the accompanying decrease in the absorbe ncy at 412 nm. VCR was degraded by increasing concentrations of HOCI in cel l-free systems and this activity was inhibited by taurine, which is known t o block HOCI activity. This finding was confirmed by the VCR cytotoxicity s tudies on cell lines. The HOCI-producing myeloblasts from patients were res istant to VCR. In five samples out of eight HOCI was also detected extracel lularly. These results suggest that oxidation by HOCI may be the final step in VCR degradation catalyzed by MPO through its action on intracellular H2 O2 and CI.