The Ink Gland of Sepia Officinalis as Biological Model for Investigations of Melanogenesis

Abstract

Melanins and melanogenesis are currently the subject of keen interest because of their increasing biotechnological applications in a variety of processes of biomedical, industrial and cosmetic relevance. Because of the notorious difficulties in the direct investigation of melanogenesis in mammals, considerable attention has traditionally been directed to elucidate the process of pigment formation in the ink gland of Sepia officinalis, which provides a most convenient and active melanin producing system. Ultrastructural studies have shown that within ink gland cells melanin formation takes place in specific subcellular organelles termed melanosomes by oxidation of tyrosine catalyzed by tyrosinase (Ortonne et al., 1981). Recent biochemical studies carried out in our laboratory have shown that, in addition to tyrosinase (Prota et al., 1981), other enzymatic activities are involved to a various extent and with different modalities in the pigment pathway. In particular, the ink gland contains a novel enzyme called dopachrome rearranging enzyme (Palumbo et al., 1994), which catalyzes the rearrangement of dopachrome to 5,6-dihydroxyindole (DHI), and a peroxidase (Palumbo & Jackson, 1995; Gesualdo et al., 1997) which catalyzes the polymerization of DHI and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) to melanin (Fig. 1). However, whether and to what extent these enzymes co-localize in the melanogenic compartments and interact is an open question. To address this aspect, polyclonal antibodies have been raised against Sepia melanogenic enzymes and their specificity has been analyzed by immunoprecipitation and western blot analysis. Immunoelectron microscopy revealed that peroxidase has a different subcellular localization pattern from tyrosinase and dopachrome-rearranging enzyme.