Enzymes of Tyrosine Metabolism

Abstract

To date three enzyme activities in Drosophila have been described which function in tyrosine metabolism. These are phenol oxidase, dopa decarboxylase and dopamine-N-acetylase. These enzymes are relevant to several different functions, including melanization or black pigment formation, sclerotization and subsequent tanning, and possibly neural transmitter synthesis. These steps in the pathways are shown in Fig. 27. Melanin is formed by the oxidation of tyrosine to a quinone precursor of the indole subunits which make up melanin. Tyrosine is also a precursor for the as yet unidentified compound(s) which are involved in the hardening and darkening of the cuticle, enabling thg cuticle to serve as an effective exoskeleton. These steps, hardening and darkening, are closely related, and are referred to as sclerotization and tanning respectively. It is beyond our scope to consider in complete detail the extensive work that has been done on the structure and sclerotization of the cuticle. Mitchell et al. (1971) have recently described the structural changes which the cuticle undergoes during molting. Aspects of the biochemical process involved in sclerotization have been amply reviewed by Pryor (1962) and Brunet (1967). Although the details of sclerotization have not been studied extensively in Drosophila, analogies to other insects indicate some general patterns which occur during this process that may be expected to apply to Drosophila. Sclerotization involves reactions between amino groups of a cuticular protein and orthoquinones, which are produced by the action of an oxidase using dopa or some other metabolic derivative of tyrosine as a substrate. The resultant cross-linked quinone-protein structure is very rigid, and gives the insect cuticle its strength. The nature of the specific reacting protein is obscure, and details of its structure and function are unclear. The information available has been summarized by Brunet (1967). The reactant protein amino groups could be from N-terminal amino acids, or the epsilon amino groups of lysine.