The mode of action of tyrosinase.

The blood and tissues of most insects contain tyrosinase, together with a suitable substrate. When blood is shed or the tissues damaged, enzyme and substrate react and eventually produce melanin; why do they not react in the intact animal? The oldest theory was that they were held apart by some structural organization of the cytoplasm and that they only came together when this was destroyed. This explanation was first questioned by Graubard (1933) as a result of his investigations of the tyrosinase content of body-colour mutants in Drosophila. He found that he could obtain an extract containing an active tyrosinase by squashing larvae or pupae in water, but that if he ground them to a fine paste with sand most of the activity was lost. He concluded that there must be some inhibitor of tyrosinase inside the cells which was set free when they were broken open. By incubating the material for about 6 hr. in chloroform vapour the inhibitor seemed to be itself destroyed, so that grinding with sand yielded an active extract. Graubard's ideas were extended by Dennell, who suggested that the inhibitor might be a dehydrogenase system, because it was put out of action by chloroform; in later papers he confirmed this and identified the enzyme concerned more precisely. The final form of Dennell's views was summarized by Wigglesworth in the 1950 edition of his text-book as follows:' Oxidation is apparently prevented by a dehydrogenase (acting upon glucose as a hydrogen donator) which reduces the oxidation-reduction potential of the blood. There is a rise in the oxidation-reduction potential just before pupation; tyrosinase can thus act upon tyrosine with the formation of DOPA.' Dennell's theory of inhibition has been extended to Carcinus by Krishnan (1950) and to Echinoderms by Jacobson & Millott (1953). Another scheme, in which the inhibitor is supposed to be sulphydryl groups rather than glucose dehydrogenase, has been suggested for vertebrate melanophores by Figge (1940). Figge supposes that tyrosinase activity is maximal at certain oxidation-reduction potentials and that it can be inhibited by a shift from the optimal value in either direction, being almost completely inhibited by glutathione or cysteine. This theory was further elaborated by Figge & Allen (1941), who claimed that the inhibition could be removed by the action of various substances such as oestrone or methylene blue. Rothman, Krysa & Smiljanic (1946) claimed that macerated human skin contains an inhibitor of tyrosinase, which they supposed to be a sulphydryl compound. Hackman (1953) suggested that sulphur is absent from the cuticular proteins of insects because it would inhibit tyrosinase.