Biochemical characterization of the melanogenic system in the eye of adult rodents

Melanins: Skin Pigments and Much More—Types, Structural Models, Biological Functions, and Formation Routes

This review presents a general view of all types of melanin in all types of organisms. Melanin is frequently considered just an animal cutaneous pigment and is treated separately from similar fungal or bacterial pigments. Similarities concerning the phenol precursors and common patterns in the formation routes are discussed. All melanins are formed in a first enzymatically-controlled phase, generally a phenolase, and a second phase characterized by an uncontrolled polymerization of the oxidized intermediates. In that second phase, quinones derived from phenol oxidation play a crucial role. Concerning functions, all melanins show a common feature, a protective role, but they are not merely photoprotective pigments against UV sunlight. In pathogenic microorganisms, melanization becomes a virulence factor since melanin protects microbial cells from defense mechanisms in the infected host. In turn, some melanins are formed in tissues where sunlight radiation is not a potential threat. Then, their redox, metal chelating, or free radical scavenging properties are more important than light absorption capacity. These pigments sometimes behave as a double-edged sword, and inhibition of melanogenesis is desirable in different cells. Melanin biochemistry is an active field of research from dermatological, biomedical, cosmetical, and microbiological points of view, as well as fruit technology.

Increased pigmentation of iridial melanocytes in primates induced by a prostaglandin analogue

The melanocytes in the mammalian eye have been thought to produce melanin only during fetal development and in the very young individual. The recent discovery that latanoprost, a prostaglandin analogue used in the treatment of glaucoma, causes increased pigmentation of the iris in monkeys and in humans indicates that the iridial melanocytes can produce melanin in adult individuals. Using microautoradiography of HG-(3)H-methimazole, a false melanin precursor, we observed in an earlier study that there seems to be an ongoing melanogenesis in adult mice in the iridial melanocytes and in the iridial pigment epithelium. In the present study latanoprost (13,14-dihydro-17-phenyl-18,19,20-trinor-PGF(2alpha)-isoprop yl ester) was applied once daily to the right eye of seven cynomolgus monkeys; the left eye served as an untreated control. Two animals developed clear-cut increased pigmentation of the iris in the treated eye during the first three months of treatment. These animals were injected intravenously with G-(3)H-methimazole and were killed 24 hr after the injection. The eyes were removed, fixed in 4% formalin supplemented with 10% acetic acid and embedded in paraffin or Polybed 812. Sections from the eyes were used for microautoradiography and light microscopic examination. A high uptake of radioactivity was observed in a few melanocytes in the iris of the untreated eye. There were also a low uptake in the melanocytes in the stroma of the ciliary body and the choroid. No accumulation was observed in the iridial or retinal pigment epithelium. In the iris of the treated eye the only observed difference from the untreated eye was an increased amount of melanin in the iridial melanocytes and an increased uptake of radioactivity in a great number of these cells. Thus it seems likely that treatment with latanoprost in some individuals causes an increase of the low normal melanin synthesis in iridal melanocytes.

Age-related melanogenesis in the eye of mice, studied by microautoradiography of 3H-methimazole, a specific marker of melanin synthesis

Whether melanogenesis occurs in adult eyes is still a matter of controversy. It has been widely held that the pigment epithelial cells are fully melanized at birth, and that the uveal melanocytes cease their melanin production in the very young individual. Therefore there should be no turnover of melanin in the adult eye. A number of studies have, however, demonstrated that the enzyme involved in melanin synthesis, tyrosinase, seems to be active also in the adult eye. The recent observation that a prostaglandin analogue, used in glaucoma therapy, caused increased iridal pigmentation in the treated eye, but not in the untreated eye, of adult monkeys and in humans, indicate that the adult eye at least has the capacity to produce melanin. In the present study 3H-methimazole, a false melanin precursor, was administered to a series of DBA-mice, 3 weeks to one year of age. The eyes were removed 24 hr after a single i.p. injection of 3H-methimazole. Using microautoradiography the incorporation of radioactivity was studied in X-ray film covered sections comprising the entire eye. A very selective accumulation of radioactivity was seen in uveal melanocytes and in the pigment epithelial cells in the iris and the ciliary body. The level in the retinal pigment epithelium was low in the eyes of all ages. No uptake was seen in any non-pigmented ocular tissue. The most pronounced accumulation was seen in the pigment epithelium and melanocytes in the iris of the young mice, but some activity was seen in these cells also in the older mice. The presence of immature melanosomes seen in electron micrographs from iridal pigment cells and melanocytes of one year old mice indicate that new melanosomes are formed in these cells also in adult animals. The results of this study thus strongly indicate that there seems to be an active melanin synthesis in the adult eye of the mouse, most pronounced in iridal melanocytes and in the iridal pigment epithelium.

Melanin formation in the inner ear is catalyzed by a new tyrosine hydroxylase kinetically and structurally different from tyrosinase

Detergent solubilized extracts of the cochleae of adult gerbils (Meriones unguiculatus) contain a tyrosine hydroxylase activity measurable by the radiometric method of Pomerantz. This activity is not related to Fenton-type reactions, since it is not inhibited by free radical scavengers and is heat and protease sensitive. It does not appear to be related to a peroxidase (EC 1.11.1.7) since it is neither dependent on H2O2, nor inhibited by catalase (EC 1.11.1.6). The involvement of a tyrosine hydroxylase (EC 1.14.16.2) related to catecholamine synthesis is also unlikely, since the activity is highly sensitive to 2-mercaptoethanol and is not increased by addition of tetrahydrobiopterin. The activity in crude inner ear extracts displayed an unusual maturation behaviour, with a slow activation upon aging at 4 degrees C. Fully active enzyme displayed Michaelis-Menten kinetics, with a Km for L-tyrosine of 47 microM. Cochlear tyrosine hydroxylase, but not melanoma tyrosinase (EC 1.14.18.1), was inhibited by o-phenanthroline, and was not dependent on L-DOPA as cofactor for full enzymatic activity. Crude extracts were also able to catalyze L-DOPA oxidation and melanin formation from either L-tyrosine or L-DOPA. The tyrosine hydroxylase, DOPA oxidase and melanin formation activities most probably resided in the same molecule, as suggested by inhibition studies. A tyrosine hydroxylase and melanin formation activity with identical properties was found in primary cultures of stria vascularis melanocytes. Immunochemical evidence confirmed the absence of either the tyrosinase encoded for by the albino locus, or the tyrosinase isoenzyme TRP1, encoded for by the brown locus. Conversely, an immunorreactive band of molecular weight 70 kDa was specifically recognized by a tyrosinase polyclonal antiserum in Western blot experiments. These results prove that melanogenesis in the cochlea, and likely in other extracutaneous locations such as the brain, is catalyzed by enzymatic systems different from, but related to tyrosinase.