Inhibition of Oxygen Evolution Rate in Spinach Chloroplasts by some Anilides of Substituted Pyridine-4-carboxylic Acids

Miroslav Miletín¹ (miletin@faf.cuni.cz), Martin Doležal¹, Katarína Kráľová², František Šeršeň²

¹Department of Pharmaceutical Chemistry and Drug Control, Charles University in Prague, Faculty of Pharmacy in Hradec Králové, Czech Republic
²Institute of Chemistry, Faculty of Natural Sciences, Comenius University, Bratislava, Slovak Republic

Summary

The inhibition of oxygen evolution rate (OER) in spinach chloroplasts produced by some anilides of 2-alkylsulfanylpyridine-4-carboxylic acids (I) or 2-chloro-6-alkylsulfanyl-pyridine-4-carboxylic acids (II) was studied. The IC₅₀ values concerning OER-inhibiting activity varied in the range from 4.8 to 69.1 µmol dm^-3. At comparable lipophilicity of the compounds the inhibitory activity of I was higher than that of II. The dependence of the OER-inhibiting activity on the lipophilicity of the compounds showed a quasi-parabolic course and the lipophilicity of the most active compounds was about log P = 5.0—5.5. The site of action of the studied compounds in the photosynthetic apparatus of spinach chloroplasts has been investigated using EPR spectroscopy. It was found that I and II interact predominantly with the intermediate D⁺ (Tyr_D) and in a pronouncedly less extent also with the intermediate Z⁺ (Tyr_Z), i. e. with tyrosine radicals situated on the donor side of photosystem (PS) 2 at the 161st position in D₂ and D₁ proteins. The intensive interaction of I and II with Tyr_D which is situated in less polar environment of the thylakoid membranes can be connected with the presence of hydrophobic alkylsulfanyl substituent in their molecules. The results of experiment with the artificial electron donor (diphenylcarbazide) acting in Z⁺/D⁺ intermediate showed that also some member of the photosynthetic electron transport chain between Z⁺/D⁺ and plastoquinone is partially damaged by I and II.