Enzymic Nature of the Protein Moiety of Protochlorophyllide Holochrome

Abstract

The enzymic nature of the protein moiety of protochlorophyll(ide) holochrome was studied by following the fate of the [14C]protochlorophyll(ide) formed when dark-grown barley (Hordeum vulgare) or bean (Phaseolus vulgaris) leaves are incubated in the dark with 3 mM 4-.delta.-[14C]aminolevulinic acid. The concentration of protochlorophyll(ide) and its specific radioactivity increases with the time of incubation. The chlorophyll formed after a single short illumination, given to the etiolated leaves at different times during their incubation with 4-.delta.-[14C]aminolevulinic acid, is 14C-labeled, and its specific radioactivity is equal to that of the total protochlorophyll(ide). The holochrome-bound protochlorophyll(ide) extracted with buffer from the so treated etiolated leaves is also 14C-labeled, and its specific radioactivity is equal to that of the total protochlorophyll(ide). Since turnover of protochlorophyll(ide) was not observed, there must be a free exchange between the old "endogenous" and the new .delta.-aminolevulinic-acid-induced protochlorophyll(ide) molecules on the active site of the holochrome protein. These results are consistent with the hypothesis that the holochrome protein acts as an enzyme.