A prohormone refers to a committed precursor of a hormone, usually having minimal hormonal effect by itself but rather circulating in the blood stream as a hormone in an inactivated form, ready to be "switched on" (activated) later by post-translational modification. The term has been used in medical science since the middle of the 20th century. Examples of natural, human prohormones include proinsulin and pro-opiomelanocortin.
For peptide hormones, the conversion process from prohormone to hormone (pro-protein to protein) typically occurs after export to the endoplasmic reticulum and often requires multiple processing enzymes. Proamylin, which is cosecreted with proinsulin, this requires the above three factors and an amidating monoxygenase. Though commonly misdescribed as such, Vitamin D3 is not an example of a prohormone or a hormone. The word prohormones has taken on a new sense due to the presence of specific oral medications designed for athletes to affect hormone levels. For small molecule hormones, the conversion is often one step, and is often used to regulate hormone levels.
- Reinhold Vieth, Why “Vitamin D” is not a hormone, and not a synonym for 1,25-dihydroxy-vitamin D, its analogs or deltanoids, Journal of Steroid Biochemistry & Molecular Biology 89–90 (2004) 571–573 http://www.direct-ms.org/pdf/VitDVieth/Vit%20D%20not%20a%20Hormone%20Vieth.pdf