Catalyzes the 6-electron oxidation of protoporphyrinogen-IX to form protoporphyrin-IX. {ECO:0000269|PubMed:21048046, ECO:0000269|PubMed:23467411, ECO:0000269|PubMed:7713909}. This is the function of Ensembl gene identifier ENSG00000143224 (PPOX, protoporphyrinogen oxidase).