Abstract

The reduction of methemoglobin by NADPH-flavin reductase of human erythrocytes through flavin was studied under various conditions using a reconstituted methemoglobin reductase system. The reduction of methemoglobin by the reconstituted enzyme system could be easily detected with flavin at the physiological concentration (e.g., 0.1-1.0 microM), and the rates obtained with 0.1 and 1.0 microM FMN were 0.19 and 2.2 nmol heme reduced per min per ml, respectively, in the absence of oxygen. FMN was more effective than FAD in reduction by the reconstituted enzyme system, and oxygen decreased the rate of the reduction. The reduction of methemoglobin by the reconstituted enzyme system with flavin at a physiological concentration proceeded as a zero order reaction. These results apparently suggest that the NADPH-flavin reductase system is able to reduce methemoglobin in erythrocytes at a moderate speed with about 1 microM flavin, and the reduction was estimated to vary from less than 1% to about 20% of that by the NADH-cytochrome b5 reductase system with 1 microM cytochrome b5, depending on the uptake of flavin by human erythrocytes.