Studies on Nε(γ-glutamyl)lysine isodipeptide in the winter flounder (Pseudopleuronectes americanus)

The results of several experiments on the synthesis of Nε(γ-glutamyl)lysine isodipeptide in the winter flounder (Pseudopleuronectes americanus) are reported. Plasma levels of the isodipeptide were low in sexually immature male fish and very high in mature male fish at spawning time. Various tissues of mature fish were examined for isodipeptide content and highest levels were found in the scales and external mucous layer. Preliminary experiments using [14C]lysine with intact scales indicated de novo synthesis of the isodipeptide in the scale preparation. The annual pattern of the levels of the isodipeptide was determined in the plasma of hypophysectomized winter flounder and sham-operated controls. Plasma levels of the isodipeptide in the control fish rose prior to spawning in May and dropped off sharply after spawning was completed. This pattern was not observed in the hypophysectomized fish. A related flatfish species, the American plaice (Hippoglossoides platessoides), had only very low levels of the isodipeptide present at spawning time, indicating that the isodipeptide is probably specific to the winter flounder.