Integral membrane protein that functions as a NADPH-dependent ferric-chelate reductase, using NADPH from one side of the membrane to reduce a Fe(3+) chelate that is bound on the other side of the membrane. Mediates sequential transmembrane electron transfer from NADPH to FAD and onto heme, and finally to the Fe(3+) chelate (PubMed:30337524). Can also reduce Cu(2+) to Cu(1+) (By similarity). Plays a role in systemic metabolic homeostasis, integrating inflammatory and metabolic responses (By similarity). Associated with obesity and insulin-resistance (PubMed:18381574, PubMed:18430367). Involved in inflammatory arthritis, through the regulation of inflammatory cytokines (PubMed:19660107). Inhibits anchorage- independent cell proliferation (PubMed:19787193). {ECO:0000250|UniProtKB:Q923B6, ECO:0000269|PubMed:18381574, ECO:0000269|PubMed:18430367, ECO:0000269|PubMed:19660107, ECO:0000269|PubMed:19787193, ECO:0000269|PubMed:30337524}. This is the function of STEAP4 (STEAP4 metalloreductase, Ensembl gene identifier ENSG00000127954).