Broad specificity oxidoreductase that catalyzes the NADPH-dependent reduction of a variety of flavins, such as riboflavin, FAD or FMN, methemoglobin and PQQ (pyrroloquinoline quinone). Contributes to heme catabolism and metabolizes linear tetrapyrroles (By similarity). NADP(+) + reduced riboflavin = 2 H(+) + NADPH + riboflavin Optimum pH is 5.0 for NADPH-methemoglobin reductase activity, and 6.5 for NADPH-diaphorase activity. Monomer. Detected in erythrocytes (at protein level).