In vitro biosynthesis of γ-lipotropic hormone

Abstract

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