Racemization of alpha-melanotropin.
Racemization of the amino acid residues of alpha-melanotropin was measured after exposure of the peptide to alkali for various lengths of time. Rates of racemization were then compared to the rate of transformation by alkali of alpha-melanotropin into a hormone with prolonged melanotropic activity. When in vitro prolongation became maximal, serine, methionine, histidine, phenylalanine and arginine were racemized 50-70%, glutamic acid, tyrosine and tryptophan 30-40% and lysine, proline and valine 10% or less. Racemization of a particular amino acid residue in alpha-melanotropin could not be associated with induction of prolongation of activity. Rather, partial racemization at multiple sites in the molecule seems almost as effective as extensive or total racemization of a single residue in producing a hormone with prolonged biological effects.