Phenol oxidase is the enzyme responsible for hardening and pigmentation of insect cuticle. The cuticular phenol oxidase is not extractable, thus unavailable for biochemical studies. The properties and composition of the enzyme, produced in vitro from extracts of late third instar Drosophila larvae, were investigated to elucidate its in vivo function.
Conditions for in vitro activation and isolation of the enzyme were studied in detail. The conditions of activation affect both the aggregation state and the density of the resulting phenol oxidase. The enzyme was isolated from the activation extract by centrifugation on sucrose gradients.
The active phenol oxidase is an insoluble, lipoprotein complex which can be partially separated into a tyrosinase and a dihydroxy phenol oxidase. The complex is largely dissociated by strong protein denaturing agents, though some covalent cross linking is indicated. Analysis of the dissociated complex by SDS acrylamide gel electrophoresis shows a large number of protein bands. The significance of these bands is discussed in relation to the purity of the phenol oxidase and expected subunit composition.
The effects of denaturing agents, urea and sodium deoxycholate, on the phenol oxidase indicate a complex relationship between enzyme structure and activity.A tentative structural model for the phenol oxidase complex isproposed.
The characteristics of in vitro phenol oxidase are discussed with regard to possible regulation of in vivo function.
