Characterization of monophenolase activity of polyphenol oxidase from iceberg lettuce |
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| Authors: | Chazarra S García-Carmona F Cabanes J |
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| Institution: | Departamento de Bioquímica y Biología Molecular A, Unidad Docente de Biología, Edificio de Veterinaria, Universidad de Murcia, Spain. |
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| Abstract: | Polyphenol oxidase (EC 1.14.18.1), a thylakoid membrane-bound enzyme, was isolated by sonication of osmotically shocked chloroplasts from iceberg lettuce (Lactuca sativa). The enzyme showed monophenolase activity when assayed on (p-hydroxyphenyl)propionic acid with 3-methyl-2-benzothiazolinone hydrazone in a reliable continuous spectrophotometric method, with high sensitivity, accuracy, and precision. The monophenolase activity showed a lag period before the steady-state rate (V(ss)) was reached. Both kinetic parameters, the lag period and the steady-state rate, depended on the pH, the enzyme and substrate concentrations, and the presence of catalytic amounts of o-diphenol. This activity shows inhibition by high substrate concentration. The experimental results correspond with the mechanism previously described for PPO from other sources. Kinetic constants K(m), V(max), and K(i) were determined. |
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