D I S C U S S I O N
In opening the discussion on Dr Al l e n ' s paper, Co p e emphasized that there seem to be two different types of free radical formed in melanin. One is stable over hours, produced by u.V., and he suspected that it is formed in the bulk of the melanin. The second is labile, its life being over in a few seconds, is produced by visible radiation and production at the face of the particle is suspected. The radical is metabolically active. He pointed out that his work was concerned almost entirely with the labile radical which involved the photo- sensitizing of choroid mitochondrin, and the interaction with catechol amines. The generation and decay of these radicals was thought to be a non-enzymatic solid state process. The decay process did not fit well with simple mass action schemes. Also radical reversibility persists on freezing, and the process is not affected by boiling of the suspension.
The Elovich equation which has been commonly observed in studies of inorganic semi-conductor processes is conformed to by the decay curve of photogenerated particles. He concluded by proposing that melanin radical generation and decay is due to photoadsorption of molecular oxygen with spontaneous desorption in the dark. This process involves the reversible transfer of an electron from an un- bound electron pair, to oxygen, the remaining unpaired electron being stabilized by resonance in the melanin molecule.
Bl o i s stated that since Co m m o n e r ' s first report 10 years ago the existence of spin-resonance absorption in melanin had been amply confirmed. Unpaired electrons certainly exist in melanin and the question of the moment is one of origin. He dismissed the theory of a semi-conductor state by quoting the Curie Law behaviour of the resonance, and added that the stability of paramagnetism against oxidation contra-indicates extensive conjugations.
Re d h a r d t enquired as to the line shape assumed in the experiments.
If the observed total line shape is caused by inhomogenous broadening the numerical value of Tx determined from saturation measurements depends on the line shape of the individual components. Al l e n
replied that whether or not the line shape is inhomogenous is un- certain. The saturation measurements made by Bl o i s must have assumed it to be homogenous unless Tx is short. This could be checked by determining line shape for different microwave powers.
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392 L I G H T A N D M E L A N I N S K I N P I G M E N T A T I O N
Me y e r complimented Fi t z p a t r i c k and Pa t h a k on their work. He commented that such studies had been in progress in France, Switzer- land and Germany since 1925 and that Sa i d m a n had demonstrated the selective absorption of u.v. by the stratum corneum, the role of pigment, reflection, and perpendicular irradiation in the effect of radiant energy on the skin.
Wi s k e m a n n asked what doses of u.v. were used to determine the action spectrum of immediate pigment darkening. Pa t h a k replied that 16-19 min of irradiation were used which was equal to i o7- i o8 ergs cm~2.
Fi t z p a t r i c k asked v a n d e r Le u n whether the differences in erythemal response obtained, which differed from his own experi- ments, might be due to absence of melanin granules in the stratum malpighii and stratum corneum; apparently no protection was afforded unless melanin was present in the malpighian layer. He thought the relative roles of stratum corneum and melanin could not be fully assessed until comparisons had been made of visible colour, light and electron microscopy of different skins. Va n d e r Le u n agreed that this was so. Pa t h a k emphasized that protection of the skin from u.v. was a function of the amount and distribution of melanin in the stratum corneum and malpighian cells. He cited his work with mono- chromatic 300 nm radiation on vitiligo, medium pigmented Caucasian and highly pigmented Negro subjects.
El He f n a w i questioned the age of onset of freckling in susceptible subjects as compared with that of only 1 year in cases of Xeroderma pigmentosum, and Nix asked whether the granule with ballooning at one end, characteristic of the Langerhans cell, was found only in cells in the upper layers of the epidermis, or also in the clear cells adjacent to the basement membrane. Br e a t h n a c h replied that the earliest age of onset in normal freckling was about 2 years, and that the granules described did occur in the lower epidermis but in fewer numbers.
