EVALUATION OF IN VITRO MELANOCYTE-DARKENING ACTIVITIES OF L-HISTIDYL-L-PHENYLALANYL-L-ARGINYL-L-TRYPTOPHYL-GLYCINE AND ITS NINE STEREOISOMERS IN RANA NIGROMACULATA

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It is a well known phenomenon that brief heating of the crude extract of pituitary gland in dilute alkali-solution potentiates its melanocyte-darkening activity and prolongs the duration of its action. This phenomenon has been investigated by two groups of investigators (1-4) using homogeneous MSHs and ACTH with which MSH activity of pituitary extract is shared. It was found that heating in alkali invariably induced racemization of the constituent amino acids of these hormones and the modified actions of alkali-treated hormones were explained in terms of their racemized structures. Yajima et al. have recently accomplished the synthesis of nine stereoisomers of L-histidyl-L-phenylalanyl-L-arginyl-L-tryptophyl-glycine (all-L) which was suggested by Hofmann et al. (5) as the active site of these hormones. In this paper, details of our in vitro assay method using Rana nigromaculata H. and the activities of the nine stereoisomers estimated by this method will be given. A certain part of this experiment has already been appeared (6).