The CONTROVERSIES featured in the August 1998 issue of Exp Dermatol have discussed the controls of melanogenesis (Schallreuter et al., Exp Dermatol 7: 143-150, 1998). Now, we explore the biological functions of the endproduct of melanogenesis, the various melanin biopolymers. As delineated in the subsequent contributions, melanins are func- Viewpoint 1Melanins are found in highly oxidizing situations where significant concentrations of oxygen radicals are generated. Such reactive oxygen species (ROS) are produced by UV-light in the skin and eye, by sound and ultrasound in the inner ear, and in the highly oxidizing conditions of catecholaminergic neurons in the brain. The role of the melanins as oxygen radical trapping polymers, UV-light filters, polymeric buffers for transition metals, calcium and catecholamines will be considered in this viewpoint. For the Dermatologist it is of special interest to understand those factors that control melanogenesis in the skin and hair (3). In this context the pterins, alpha melanocyte stimulating hormone (a-MSH) and calcium appear to be central in the regulation of both melanin content and composition in melanocytes. There is accumulating evidence that the evolution of melanogenesis in human melanocytes is primarily antioxidant in character and is designed to protect these pigment cells from the cytotoxicity of ROS. However, the subtle link between calcium homeostasis and melanogenesis suggests that natural skin colours in the human population may have evolved secondarily as a consequence of salt balance/dehydration fac-153 tionally much more complex and fascinating than their evident colour-awarding and UV-light-filtering properties suggest, and dermatologists and pigment biologists alike have yet to discover and define the full range of biologically relevant melanin functions in health and disease.tors as primates adapted to extreme differences in local climates. It is tempting to speculate that skin colour emerged as a side effect through Darwinian natural selection principles in tropical versus temperate climates.There is increasing evidence that melanogenesis represents a major antioxidant defence mechanism in melanocytes. Melanocytes in the human epidermis (in vivo) or in cell cultures (in vitro) have been shown to be much more sensitive to the cytotoxicity of ROS than keratinocytes (17, 27). This concept is especially well-documented by the cytotoxicity of hydrogen peroxide (H 2 O 2 ) to melanocytes both (in vivo) and (in vitro) (6,14,15,17). Consequently, melanocytes have evolved an effective strategy to deal with the removal of superoxide anion radicals (O 2 ª ) before these radical anions disproportionate to H 2 O 2 .The preferential utilization of O 2 ª compared to dioxygen (O 2 ) is quite a common event in the skin. The enzymes tyrosinase, proline hydroxylase and indoleamine 2,3 dioxygenase selectively utilize O 2 ª as the preferred substrate over O 2 for the formation of dopaquinone, hydroxyproline residues in collagen, and kynurenic acid respectively (2, 20, 21,...

Spectra of ultraweak chemiluminescence (CL) accompanying auto-oxidation and hydration of cereal products have been measured using single photon counting and cut-off filters. The spectra cover the 380-880 nm spectral range with maxima centred around 600 nm. Analytically pure air-dried carbohydrates like agar, cellulose and nitrocellulose give emission too weak for spectral measurements. The emission from water pure carbohydrates is on average 4-12 times higher and emission spectra are similar to those from cereal products. The effect of free radical scavengers, SOD and O2* (1 delta g)-quenchers on CL spectra indicates a contribution of radical reactions with the participation of excited carbonyls, O2- and excited molecular oxygen dimoles. Moreover, possible mechanisms of chemi-excitation due to a cooperative H-bond formation during the hydration of carbohydrates and/or recombination of trapped radicals and electron-holes are discussed. It is also postulated that the excitation energy transfer to natural sensitizers occurring in cereal products may account for non-specific broad spectra and differences in the intensity of CL.

Formaldehyde and gallic acid oxidized with aqueous alkaline hydrogen peroxide produce relatively strong chemiluminescence

The fundamental laws of photochemistry and the essential results of experimental research on ultraweak cell radiation are presented. By comparing all the facts it can be concluded that the phenomena discussed may arise from a variety of possible reactions and sources. Recombination reactions of certain radicals actually do release sufficient energy to generate UV-photons of the intensity under consideration. On the other hand, stimulated emission cannot be excluded in view of the distinct deviation of the radiation field from thermal equilibrium. There exist, however, various other candidates, such as direct emitters like flavins, indoles, porphyrins, carbonyl derivatives and aromatic compounds, and molecular oxygen and its various species, as well as collective molecular interactions, e.g. dimole or exciplex transitions, triplet-triplet annihilation, collective hydrolysis, electric field effects in membranes, etc. Careful biochemical and biophysical experiments are still necessary to find answers to all the questions that remain; not only individual problems have to be solved, but it is important to keep in mind the interrelationships between certain reactions.