Three types of alpha-melanocyte-stimulating hormone: bioactivities and half-lives

Three types of alpha-melanocyte-stimulating hormone (alpha MSH) that differ in the acetyl status of the N-terminal serine have been found in the neurointermediate lobe of the pituitary gland and in the brain: desacetyl alpha MSH, which lacks an acetyl group; monoacetyl alpha MSH, in which the amino group of the serine is acetylated; and diacetyl alpha MSH, in which both amino and hydroxy groups of the serine are acetylated. We compared the lipolytic and melanotropic actions of these three peptides, and their rates of disappearance from plasma, both in vitro and in vivo. The following differences were found. a) For in vitro lipolytic actions on rabbit adipose tissue slices, the potencies differed according to the order diacetyl = monoacetyl greater than desacetyl. On rabbit isolated adipocytes, however, the three peptides were equipotent. b) For in vivo lipolytic action in the rabbit, not only potency but also kinetics differed. Diacetyl alpha MSH had the slowest onset, longest duration, and greatest potency. The desacetyl variant had the quickest onset, shortest duration, and least potency. c) The half-life for elimination from rabbit plasma both in vitro and in vivo was shortest for the desacetyl form and longest for the diacetyl peptide. d) For in vitro melanotropic effect on frog skin, kinetics of action were the same for all three peptides, but potency differed according to the order diacetyl = monoacetyl greater than desacetyl. Thus acetylation of alpha MSH alters lipolytic and melanotropic potencies in vitro and lipolytic potency and kinetics in vivo. These differences result in part from the fact that acetylation slows the degradation of the tridecapeptide both inside and outside the circulation.