Abstract
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 H(2)S 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 N(2), 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 H(2)S is less than 3 minutes in duration; longer periods reduce the reversion capacity proportionately. The leaves are relatively undamaged by brief exposures to H(2)S, 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 HN(3) cause an irreversible conversion of protochlorophyll(ide)(650) to protochlorophyll(ide)(633) with total loss of photoactivity. Sulfhydryl agents, such as beta-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 H(2)S 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.