Cytochrome b5, Form B (CYB5B)

Cytochromes b5 are ubiquitous electron transport hemoproteins found in animals, plants, fungi and purple phototrophic bacteria. The microsomal and mitochondrial variants are membrane-bound, while bacterial and those from erythrocytes and other animal tissues are water-soluble. Two isomers of cytochrome b5, referred to as the A (major) and B (minor) forms, differ by a 180° rotation of the heme about an axis defined by the α- and γ-meso carbons. The deduced 153-amino acid protein shares 45.8% homology with CYB5A, and RT-PCR analysis showed CYB5B expression in human liver, testis, and adrenal tissues. CYB5B had 17,20-lyase activity as measured by the conversion of pregnenolone to dehydroepiandrosterone (DHEA) when expressed in HEK293 cells.