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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Abstract
When the potency of the all-L pentapeptide was taken as 1.0, relative potencies of isomers were as follows: L-His-D-Phe-L-Arg-L-Try-Gly 1.6x102 >L-His-L-Phe-L-Arg-D-Try-Gly 1.7 x 10 > L-Hls-D-Phe-D-Arg-L-Try-Gly 1.0 x 10 > L-His-D-Phe-D-Arg-D-Try-Gly 2.1 > D-His-L-Phe-L-Arg-D-Try-Gly 1.2 > all-L 1.0 > D-His-D-Phe-D-Arg-L-Try-Gly 0.7> D-His-L-Phe-L-Arg-L-Try-Gly and L-His-L-Phe-D-Arg-L-Try-Gly nearly 0> D-His-D-Phe-D-Arg-D-Try-Gly 0. In the molecule of the pentapeptide, D-configuration of histidine or arginine residue invariably reduced activity, while D-configuratlon of phenylalanine residue led to a high increase in activity. On the other hand, the last three peptides being Inactive or low active by themselves, were found to reverse the darkening action of the all-L pentapeptide and that of a-MSH. But the action of caffeine was not influenced by these peptldes. This finding seems to suggest that caffeine marked?y differs from MSH[melanocyte stimulatory hormone]-related peptides in its mode of action. Assay results obtained using Rana nigromaculata differed from those reported by other Investigators who used the melanocyte of Rana piplens for assay. The species difference seems to be responsible for this discrepancy.