title: Characterization of monophenolase activity of polyphenol oxidase from iceberg lettuce.
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.
authors.affiliation: Departamento de Bioquímica y Biología Molecular A, Unidad Docente de Biología, Edificio de Veterinaria, Universidad de Murcia, Spain.
authors.forename: S
authors.initials: S
authors.lastname: Chazarra
authors.forename: F
authors.initials: F
authors.lastname: García-Carmona
authors.forename: J
authors.initials: J
authors.lastname: Cabanes
mesh_terms: D004156:Catechol Oxidase / Q000378:metabolism*; D007700:Kinetics; D018545:Lactuca / Q000201:enzymology*; D010088:Oxidoreductases / Q000378:metabolism*; D012680:Sensitivity and Specificity; D013053:Spectrophotometry / Q000379:methods
chemical_list: D010088:Oxidoreductases; D004156:Catechol Oxidase; C412191:monophenolase