The molecular background of Hg (2+)-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 Hg (2+) completely inhibits the activity of NADPH : Pchlide oxidoreductase ( ). 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 Hg (2+) can also react directly with Pchlide, resulting in protopheophorbide formation. At 10 (-3) M Hg (2+), 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 Hg (2+) effect: depending on concentration it can react with any components of the NADPH : Pchlide oxidoreductase macrodomains.
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