Hill Reaction in Pea Chloroplasts: Contribution of Photosystem II and Photosystem I to Ferricyanide Reduction

V. D. Samuilov,^1,2 E. L. Barsky,¹ and A. V. Kitashov¹

¹Department of Cell Physiology and Immunology, Lomonosov Moscow State University, Moscow, 119899 Russia; fax: (095) 939-3807.

²To whom correspondence should be addressed.

Submitted March 19, 1997.

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The effect of DNP-INT (an inhibitor of cytochrome b₆f complex at the level of plastoquinol Q_ZH₂ oxidation) on O₂ evolution by pea chloroplasts in the Hill reaction with ferricyanide as an electron acceptor was studied. The DNP-INT inhibitory effect on O₂ evolution decreased with a decrease in light intensity. This indicates that ferricyanide is preferably reduced by photosystem (PS) II than PS I at low light. The inhibitory effect of DNP-INT in the Hill reaction with ferricyanide at saturating and non-saturating light intensity was not affected by antimycin A, an inhibitor of PS I-dependent cyclic electron transfer at the level of ferredoxin-plastoquinone reductase. The inhibitory effect of DNP-INT on electron transfer from H₂O to ferricyanide decreased with an increase in ferricyanide concentration from 50 µM to 3 mM: the contribution of PS II to ferricyanide reduction increased with the increase of ferricyanide concentration. The effect of DNP-INT on the photoreduction of other Hill reagents (silicomolybdate, phenyl-p-benzoquinone, p-benzoquinone, duroquinone, vitamin K₃, DPIP, TMPD, and DAD) was also investigated.

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KEY WORDS: electron transfer, photosystem II, photosystem I, ferricyanide, DNP-INT, antimycin A, chloroplasts.

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