The Conversion of Photoinactive Protochlorophyllide633 to Phototransformable Protochlorophyllide650 in Etiolated Bean Leaves Treated with δ-Aminolevulinic Acid

Abstract

The relationship of phototransformable protochlorophyllide to photoinactive protochlorophyllide has been studied in primary leaves of 7- to 9-day-old dark-grown bean (Phaseolus vulgaris L. var. Red Kidney) seedlings. Various levels of photoinactive protochlorophyllide, absorbing at 633 nm in vivo, were induced by administering delta-aminolevulinic acid to the leaves in darkness. Phototransformable protochlorophyllide, absorbing at 650 nm in vivo, was subsequently transformed to chlorophyllide by a light flash, and the regeneration of the photoactive pigment was followed by monitoring the absorbance increase at 650 nm in vivo. A small increase in the level of protochlorophyllide(633) causes a marked increase in the extent of regeneration of protochlorphyllide(650) following a flash. High levels of the inactive pigment species, however, retard the capacity to reform photoactive protochlorophyllide. A nonstoichiometric and kinetically complex decrease in absorbance at 633 nm in vivo accompanied the absorbance increase at 650 nm. The half-time for protochlorophyllide(650) regeneration in control leaves was found to be three times longer than the half-time for conversion of chlorophyllide(678) to chlorophyllide(683) at 22 C. The results are consistent with the hypothesis that protochlorophyllide(633) is a direct precursor of protochlorophyllide(650) and that the protein moiety of the protochlorophyllide holochrome acts as a "photoenzyme" in the conversion of protochlorophylide to chlorophyllide.