The Role of N ADPH in the Reversible Phototransformation of Chlorophyllide P682 into Chlorophyllide P678 in Etioplasts of Oat

Abstract

Abstract Absorption spectra and room temperature fluorescence variations upon light excitation were studied in etioplasts in which the protochlorophyllide had been reduced to chlorophyllide prior to the experiment. In the presence of 1 mᴍ NADPH, the absorption band was located around 682 nm and the fluorescence variations reflected the occurrence of the chlorophyllide microcycle (reversible phototransformation of chlorophyllide P682 into chlorophyllide P678), whereas, in the absence of NADPH, the absorption band was located around 676 nm and the fluorescence exhibited only a slow decrease with illumination time. After washing, NADPH-treated etioplasts still exhibited an absorption maximum of 682 nm, which shifted irreversibly to 678 nm upon illumination. Re-addition of only 10 μᴍ NADPH restored the reversibility of the reaction. Plastids were also prepared after a brief illumination of the intact leaves. These plastids contained the P682 chlorophyllide. In this preparation, essentially the same phenomena were observed as in NADPH-treated etioplasts after washing. Moreover, enzymic oxidation of NADPH resulted in the transformation of P682 into P678 in darkness. Addition of the Hill electron acceptors 2,6- dimethylbenzoquinone on FeCN caused an acceleration of the phototransformation of P682 into P678. We interpret these results as being due to the oxidation of NADPH during the phototransformation of P682 into P678. The nature of the electron acceptor and the physiological role of this reaction is discussed.