N , 0 - Diacetylserinel a - Melanocyte - stimulating Hormone , a Naturally Occurring

When aqueous extract of bovine pituitary gland was fractionated by high pressure liquid chromatography (HPLC), the effluent contained a prominent peak of melanotropic activity which did not correspond to amelanocyte-stimulating hormone (MSH), /?-MSH, adrenocorticotropic hormone, or p lipotropin. The novel factor (labeled “post-a-MSH”), responsible for onethird of the melanotropic activity in the gland, was isolated by acetic acid extraction, fractional precipitation with organic solvents, and HPLC. Mobility of postwMSH was the same as of a-MSH in zone electrophoresis and paper and thin layer chromatography (TLC), but differed on reverse phase TLC. Amino acid compositions of acid hydrolysates of post-a-MSH and aMSH were identical. Melanotropic and lipolytic potencies were also the same. Post-a-MSH contained neither glutamine nor carbohydrate. As in a-MSH, both termini were blocked. When post-a-MSH was incubated at pH 9 to 12, it spontaneously converted to a-MSH. HPLC analyses of fragments obtained after enzymatic digestions of both peptides showed that post-a-MSH was a structural variant of a-MSH with N,O-diacetylserine as its NHz-terminal residue. Post-a-MSH is responsible for about one-third of the melanotropic activity in aqueous extracts of cattle, rat, guinea pig, or rabbit hypophysis. When the gland is incubated in vitro, >90% of the melanotropic activity released represents post-a-MSH. Physiologic secretion of a-MSH, therefore, may be related to acetylation of the hydroxy group of serine 1. Peptide IIF, a previously described melanotropic preparation from choroid plexus, was found by HPLC to contain 1% a-MSH and 99% melanotropically inactive material.