The fine structure of integumental erythrophores and the intracellular location of pteridine and carotenoid pigments in adult goldfish, Carassius auratus, were studied by means of cytochemistry, paper and thin-layer chromatography, ionophoresis, density-gradient centrifugal fractionation, and electron microscopy. The ultrastructure of erythrophores is characterized by large numbers of somewhat ellipsoidal pigment granules and a well-developed system of tubules which resembles endoplasmic reticulum. The combined morphological and biochemical approaches show that pteridine pigments of erythrophores are located characteristically in pigment granules and are the primary yellow pigments of these organelles. Accordingly, this organelle is considered to be the "pterinosome" which was originally found in swordtail erythrophores. Major pteridines obtainable from goldfish pterinosomes are sepiapterin, 7-hydroxybiopterin, isoxanthopterin, and 6-carboxyisoxanthopterin. Density-gradient fractions indicate that carotenoids are mostly associated with the endoplasmic reticulum. Both tyrosinase and possibly a tyrosinase inhibitor containing sulfhydryl groups are present in the pterinosome. The possible existence of a tyrosinase inhibitor is suggested by the marked increase of tyrosinase activity upon the addition of iodoacetamide or p-chloromercuribenzoic acid. In the light of their fine structure, pigmentary composition, and enzymatic properties, the erythrophores and pterinosomes are discussed with respect to their probable functions and their relationship to melanophores.

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