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Photoreduction of Acceptor Generated in an Ascorbate Peroxidase Reaction in Pea Thylakoids

B. N. Ivanov,1,2 L. K. Ignatova,1 V. I. Ovchinnikova,1 and S. A. Khorobrykh1

1Institute of Soil Science and Photosynthesis, Russian Academy of Sciences, Pushchino, Moscow Region, 142292 Russia; fax: (0967) 79-0532; E-mail: ivabor@issp.serpukhov.su

2To whom correspondence should be addressed.

Submitted March 31, 1997; revision submitted May 29, 1997.
Characteristics of the ascorbate peroxidase reaction and photoreduction of monodehydroascorbate (MDHA) radical generated in this reaction were investigated in pea thylakoids. Ascorbate at millimolar concentrations was necessary for thylakoid-bound ascorbate peroxidase (tAPX) protection from inhibition by hydrogen peroxide. The Km for H2O2 in the ascorbate peroxidase reaction was independent of the pH in the range from 7.0 to 8.0 and was 17 µM. At pH 8.0, the Km for H2O2 in light-dependent oxygen evolution which resulted from H2O2 addition to thylakoids was close to the Km for H2O2 in the ascorbate peroxidase reaction. At pH 8.0 and saturating H2O2 the rates of these reactions were nearly equal, indicating that under such conditions the rate of electron transport depended on the rate of MDHA generation in the ascorbate peroxidase reaction. The rate of the latter reaction changed insignificantly while the rate of oxygen evolution 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 influence of MDHA disproportion on such a supply. The results implied the possibility of a competition between MDHA radical and Mv for electrons from the electron transport chain.
KEY WORDS: ascorbate peroxidase, monodehydroascorbate, methyl viologen, electron transport, thylakoids, Pisum sativum L.