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 K_m 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 K_m for H₂O₂ in light-dependent oxygen evolution which resulted from H₂O₂ addition to thylakoids 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 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.

---