In insects, the enzyme phenol oxidase is involved in the hardening and darkening of the cuticle. Drosophila phenol oxidase occurs in a latent form. Phenol oxidase activity is produced as a consequence of the interaction of several proteins. Three of these proteins are designated as the A components (A1, A2, A3). A quantitative assay for the A components has been developed. The A1 component has been prepared in stable and highly purified form by gel filtration, preparative electrofocusing, and preparative electrophoresis. No major contaminants could be detected by analytical polyacrylamide gel electrophoresis, analytical electrofocusing, SDS gel electrophoresis, and analytical ultracentrifugation. The A1 component has a molecular weight of approximately 77,000 daltons and an isoelectric point of 5.1. The isoelectric point of the A2 component is 6.0, and its molecular weight is similar to that of the A1 component. The A1 component has no detectable phenol oxidase activity and cannot be converted to active phenol oxidase by the S (salivary gland) component. The A1 component contains approximately 0.15% copper and does not contain large amounts of fatty acids or phosphate. The amino acid composition of A1 is reported. Microheterogeneity was observed in preparations of purified A1. The relationship of the A components to active phenol oxidase and the mechanism of the activation reaction are discussed.
