Generation of reactive oxygen intermediates by human liver microsomes in the presence of NADPH or NADH.

Studies were carried out to evaluate the ability of human liver microsomes to generate superoxide radical and hydrogen peroxide, and to interact with ferric chelates to produce more potent oxidizing species such as the hydroxyl radical (.OH). In the presence of either NADPH or NADH, human liver microsomes produced superoxide and H2O2 at rates about 20 to 30% of that found with rat liver microsomes. These lower rates are caused, in part, by the 3-fold lower content of total cytochrome P450 in the human liver microsomes. NADH-dependent rates were about 25 to 30% of the NADPH-dependent rates. In the presence of appropriate ferric complexes, human liver microsomes generated .OH, promoted cleavage of vicinal diols, and underwent lipid peroxidation. In contrast to results with rat liver microsomes, NADH-dependent rates of .OH production or lipid peroxidation by human liver microsomes were similar to the NADPH-dependent rates. Human liver microsomes reduced ferric ATP or ferric EDTA at nearly comparable rates with NADPH and NADH. Sensitivity of the various iron-dependent reactions to antioxidants was found to be characteristic of the particular system. These results suggest the possibility that human liver microsomes are an important source of reactive oxygen intermediates, especially under conditions of increased NADH or NADPH availability and elevated iron concentration.