Some aspects of melanin pigmentation in amphibia

Abstract

Black and brown pigments are widely distributed in animals, and most of them are melanin. In spite of the vast amount of biological, biochemical and chemical research on melanin, it remains ill defined chemically and difficult to identify. The chemical structure of naturally occurring melanin is unknown and there is still some uncertainty about its elementary composition, because of the difficulty of extracting it.Synthetic and natural melanins have been defined by Thomson (1962) as dark coloured biochromes. They are nitrogenous polymers of high molecular weight formed by the enzymatic oxidation of phenols. This definition, however, would include a whole range of products derived by oxidation of many phenolic substrates. Thus Thomson derives melanin from tyramine and 3-hydroxy-tyramine. However, the generally accepted definition is restricted to the pigments formed by oxidation of tyrosine or dihydroxyphenylalanine (dopa) by tyrosinase. The melanin formed in vitro from tyramine and 3-hydroxytyramine is chemically different from tyrosine melanin. The melanin monomer is thought to be an indole unit, but this has not yet been proved. Natural tyrosine melanin is frequently bound to protein and the linkage is believed to occur by sulphydryl groups, between the C-4 position in the melanin and an S-atom in the protein. This work was undertaken in order to investigate the distribution of melanin in certain amphibians, and to find any possible relationship between distribution and formation of melanin, the occurrence of the enzyme tyrosinase and its substrate tyrosine, and the inhibition of melanogenesis in developing tadpoles. (Abstract shortened by ProQuest.)