Hg2+ reacts with different components of the NADPH:: Protochlorophyllide oxidoreductase macrodomains

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.