Hg²⁺ Reacts with Different Components of the NADPH : Protochlorophyllide Oxidoreductase Macrodomains

Abstract

The molecular background of Hg²⁺-induced inhibition of protochlorophyllide (Pchlide) photoreduction was investigated in homogenates of dark-grown wheat leaves. Our earlier work showed that 15 min incubation with 10^-2 M Hg²⁺ completely inhibits the activity of NADPH : Pchlide oxidoreductase ([Lenti et al., 2002]). Detailed analysis of spectra recorded at 10 K indicated the appearance of emission bands at 638 and 650 nm, which are characteristic for NADP⁺-Pchlide complexes. Fluorescence emission spectra recorded with different excitation wavelengths, fluorescence lifetime measurements and the analysis of acetone extractions revealed that Hg²⁺ can also react directly with Pchlide, resulting in protopheophorbide formation. At 10^-3 M Hg²⁺, the phototransformation was complete but the blue shift of the chlorophyllide emission band speeded up remarkably. This indicates oxidation of the NADPH molecules that have a structural role in keeping together the etioplast inner membrane components. We suggest a complex model for the Hg²⁺ effect: depending on concentration it can react with any components of the NADPH : Pchlide oxidoreductase macrodomains.

Key words

Mercury - Hg²⁺ - protochlorophyllide - NADPH - protopheophorbide - Shibata shift - activity loss

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