(A) P450 reductase activity in parental MDA468 cells and P450R transfected clones R3, R7, R12, R10, R5. P450R activity was measured using the reduction of cytochrome c. (B) Transfected P450R can function as a bioreductive enzyme in MDA468 cells. Oxygen consumption was measured in parental and P450R transfected MDA468 cells using a Clark electrode before and after treatment with 25 μM menadione. Results are expressed as the mean ± standard deviation of three separate determinations. * Significantly different from parental cells, p < 0.05, ** p < 0.01. (C-D) Correlation analysis between P450R activity and IC50 values for RH1 in various P450R transfected MDA468 clones. IC50 values were determined by the MTT assay (C) or the SRB assay (D). IC50 values are expressed as the mean ± standard deviation of at least four separate determinations. P450R activity represents the mean of at least four separate determinations. ○, parental MDA468; ▲, R3; ▼, R7; ■, R12; ●, R10; □, R5.