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

^a Department of Biological Sciences, University of Illinois at Chicago Circle, Chicago, Illinois 60680

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 -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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