CONVERSION OF CHLOROPHYLLIDE TO PHEOPHORBIDE BY MG-DECHELATING SUBSTANCE IN EXTRACTS OF CHENOPODIUM-ALBUM

Pheophorbide formation in chlorophyll metabolism was studied using ext racts of Chenopodium album. A new substance catalyzing the conversion of chlorophyllide a to pheophorbide a was found in the filtrate throug h Molcut II (Mr 5,000) of the extracts of C. album. The filtrate requi red chlorophyllide a as the sole substrate for pheophorbide a formatio n, and the boiled filtrate had complete activity, indicating that this Mg-dechelating substance is heat stable. Optimum conversion of chloro phyllide a to pheophorbide a was observed at pH 7 to 6.5 which is diff erent from chemical formation of pheophorbide a by acid. In addition t o chlorophyllide a, this substance used chlorophyllide b, bacteriochlo rophyllide a as the substrate, but not chlorophylls a and b, monovinyl -protochlorophyllide a and Mg-protoporphyrin IX dimethyl ester. The ac tivity was stimulated by the addition of chelators such as EDTA and o- phenanthroline. The maximal activity was obtained at a concentration o f 2 mM for EDTA. The activity was inhibited by the addition of Mg2+ io n; about 50% inhibition was observed at 10 mM MgCl2. These findings su ggest that the substance catalyzing the conversion of chlorophyllide t o pheophorbide, tentatively called Mg-dechelating substance, is involv ed in the breakdown of chlorophylls.