Pro-opiomelanocortin Precursor	

ACTH is derived from a large precursor molecule, pro-opiomelanocortin (POMC) (Fig. 9.7). This precursor gives rise to three molecules that contain the melanocyte-stimulating hormone (MSH) sequence: the amino-terminal sequence containing the g-MSH sequence; ACTH with a-MSH; and b-lipotrophin (b-LPH) with b- MSH peptide sequences. The relative importance of these molecules in the pigmentation that occurs in the disease is unclear. Assays have been developed for the amino-terminal sequence (N-POC) and for b-LPH. The levels of these molecules are high in Addison's disease and, since they are more stable in plasma than ACTH, they may prove to be useful in biochemical diagnosis, particularly where sample-handling facilities (e.g. cold centrifuge) are unavailable.