Cytochrome b5 deficiency is a rare condition and form of isolated 17,20-lyase deficiency caused by deficiency in cytochrome b5, a small hemoprotein that acts as an allosteric factor to facilitate the interaction of CYP17A1 (17α-hydroxylase/17,20-lyase) with P450 oxidoreductase (POR), thereby allowing for the 17,20-lyase activity of CYP17A1.
[1][2][3] The condition affects both adrenal and gonadal androgen biosynthesis and results in male pseudohermaphroditism.
[1] The principal biological role of cytochrome b5 is reduction of methemoglobin, so cytochrome b5 deficiency can also result in elevated methemoglobin levels and/or methemoglobinemia, similarly to deficiency of cytochrome b5 reductase (methemoglobin reductase).
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