PHOTOREDUCTION OF ACCEPTOR GENERATED IN AN ASCORBATE PEROXIDASE REACTION IN PEA THYLAKOIDS

Characteristics of the ascorbate peroxidase reaction and photoreductio n of monodehydroascorbate (MDHA) radical generated in this reaction we re investigated in pea thylakoids. Ascorbate at millimolar concentrati ons was necessary for thylakoid-bound ascorbate peroxidase (tAPX) prot ection from inhibition by hydrogen peroxide. The K-m for H2O2 in the a scorbate peroxidase reaction was independent oi. the pH in the range f rom 7.0 to 8.0 and was 17 mu M. At pH 8.0, the K-m for H2O2 in light-d ependent oxygen evolution which resulted from H2O2 addition to thylako ids was close to the K-m for H2O2 in the ascorbate peroxidase reaction . At pH 8.0 and saturating H2O2 the rates of these reactions were near ly equal, indicating that under such conditions the rate of electron t ransport depended on the rate of MDHA generation in the ascorbate pero xidase reaction. The rate of the latter reaction changed insignificant ly while the rate of in the response to H2O2 addition dropped when the pH was decreased from 8.0 to 7.0, These data suggest the ''tunnelling '' supply of MDHA radical to the site of its photoreduction and the in fluence of MDHA disproportion on such a supply. The results implied th e possibility of a competition between MDHA radical and My for electro ns from the electron transport chain.