A Reversible Conversion of Phototransformable Protochlorophyll(ide)656 to Photoinactive Protochlorophyll(ide)656 by Hydrogen Sulfide in Etiolated Bean Leaves 1

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The relationship of phototransformable protochlorophyll-(ide) to photoinactive protochlorophyll(ide) has been studied in the primary leaves of 7- to 9-day-old dark-grown bean (Phaseolus vulgaris L. var. Red Kidney) seedlings. Subjecting the leaves to an atmosphere of H2S causes an immediate loss of phototransformable protochlorophyll(ide)650 and a simultaneous increase in photoinactive protochlorophyll(ide)633. When such leaves are returned to air or N2, the absorbance at 650 nm increases, whereas the absorbance at 633 nm decreases and photoactivity is restored. The reversion of protochlorophyll-(ide)633 to protochlorophyll(ide)650 is one-half complete in 3 minutes at 22 C in 8-day-old leaves. Ninety-five per cent recovery of protochlorophyll(ide)650 is obtained when exposure to H2S is less than 3 minutes in duration; longer periods reduce the reversion capacity proportionately. The leaves are relatively undamaged by brief exposures to H2S, as judged by electron microscopy and by their ability to synthesize chlorophyll under continuous illumination. Hydrogen sulfide has no immediate effect upon the absorption properties of a partially purified preparation of the protochlorophyll(ide) holochrome, an etioplast suspension, or leaves subjected to freezing and thawing. Compounds such as HCN and HN3 cause an irreversible conversion of protochlorophyll(ide)650 to protochlorophyll(ide)633 with total loss of photoactivity. Sulfhydryl agents, such as β-mercaptoethanol and cysteine, cause a slow, irreversible transformation of the photoactive pigment to the photoinactive form and inhibit the ability of the leaves to synthesize chlorophyll under continuous illumination. The results obtained suggest that H2S may alter the interaction between the source of hydrogens on the protein moiety of the holochrome and the chromophore in vivo by reducing a disulfide bond in the protein, thereby causing a reversible conformational change in the complex.

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