In vitro biosynthesis of gamma-lipotropic hormone.

Sheep gamma-lipotropic hormone (gamma-LPH) is a pituitary polypeptide made of 58 amino acids and is formed of the first 58 residues of beta-lipotropic hormone (beta-LPH). The C-terminal portion (41-58) of gamma-LPH is identical with the structure of beta-melanophore-stimulating hormone (beta-MSH). We hypothetized in 1967 that beta-LPH could be the biological precursor of beta-MSH and that gamma-LPH could be an intermediate compound. We demonstrated in 1974 that beta-LPH is actively synthesized in the bovine pituitaries. We now studied the biosynthesis of gamma-LPH by monitoring the incorporation of radioactive amino acids in beef pituitary slices. We separated gamma-LPH from the other radioactive proteins with a method previously described. We characterized the radioactive proteins by ion-exchange chromatography, gel filtration and polyacrylamide gel electrophoresis. Our results show that radioactive gamma-LPH was actively synthesized. This gamma-LPH has all the chemical characteristics of nonradioactive gamma-LPH. However, in the conditions used, we were unable to demonstrate biosynthesis of beta-MSH. These results suggest that gamma-LPH is biosynthesized more slowly than beta-LPH and that the conversion into beta-MSH, if it exists, is a slow or subactive process in the species studied.