Melanins

Abstract

Melanins have been defined comprehensively as the dark brown or black pigments which accumulate in certain parts of plants and animals. On chemical grounds this definition is not satisfactory for the higher plants, as there exist strong reasons for supposing that whereas some of the dark pigments in these plants are indolic compounds, others do not contain nitrogen. Examples of non-nitrogenous pigments (see p. 662) are first the phytomelanes of the Compositae, and secondly Japanese lac and a whole list of other oxidative products of nitrogen-free phenols, seen in functioning specialized organs such as bud-scale leaves, or making their appearance when cells are injured. In the present article, even the blackest of these non-nitrogenous pigments will not be regarded as a melanin. The use of the term will be confined to dark pigments which are nitrogenous because they are indole derivatives. The definition now to be given will cover such authentic melanins as have been extracted from animal organs or produced by enzyme action in vitro. A naturally occurring melanin is a dark polymeric indole derivative of high molecular weight which is produced by a series of reactions (section F) involving tyrosinase, oxygen and, as organic substrates, either tyrosine or dihydroxyphenylalanine (“dopa”).