Chlorophyll catabolism in broccoli (Brassica oleracea L.) flower buds: An involvement of Mg-dechelatase

To elucidate the mechanism of degreening in flower buds of broccoli (Brassi ca oleracea L.), high performance liquid chromatography (HPLC) was used to separate chlorophyll derivatives in acetone extract derived from broccoli f lower buds. Low levels of chlorophyll a derivatives, i.e. chlorophyllide a, pheophorbide a, pyropheophorbide a, C13(2)-hydroxy- chlorophyll a, and phe ophytin a were detected in extracts with or without treatment with ethylene for 48hr. Chlorophyll degrading reaction, the conversion of chlorophyllide a to pheophorbide a, was catalyzed by soluble enzyme in the acetone powder extract from flower buds in the presence of chlorophyllin, Mg-chlorin prep ared from chlorophyll a as a substrate. The release of Mg2+ is associated with a shift of absorption from 658 to 68 7 nm. Changes in absorption at 687 nm are taken as a measure of Mg-dechelat ase activity. Heating the extract prevented the reaction from proceeding, i ndicating that the protein/enzyme in the extract was inactivated. Mg-dechel atase did not catalyze the releasing reaction of Cu2+ from Cu-chlorophyllin as substrate. Hence, Mg-dechelatase of the flower buds appears to recogniz e Mg-chlorin specifically. Moreover, a high negative correlation which exis ted between the total chlorophyll contents in flower buds and Mg-dechelatas e activity is expressed as: Y=-0.73X+97.37, r(2) = 0.94, where X and Y are Mg-dechelatase activity and the total chlorophyll contents, respectively. T hese findings indicate that chlorophyll degradation in the flower buds is c aused by the enhancement of Mg-dechelatase activity.