THE KEY STEP IN CHLOROPHYLL BREAKDOWN IN HIGHER-PLANTS - CLEAVAGE OF PHEOPHORBIDE-ALPHA MACROCYCLE BY A MONOOXYGENASE

Chlorophyll breakdown in green plants is a longstanding biological eni gma. Recent work has shown that pheophorbide a (Pheide a) derived from chlorophyll (Chl) is converted oxygenolytically into a primary fluore scent catabolite (pFCC-1) via a red Chl catabolite (RCC) intermediate. RCC, the product of the ring cleavage reaction catalyzed by Pheide a oxygenase, which is suggested to be the key enzyme in Chi breakdown in green plants, is converted into pFCC-1 by a reductase. In the present study, an in vitro assay comprising O-18(2) Pheide a oxygenase and RC C reductase yielded labeled pFCC-1. Fast atom bombardment-mass spectro metric analysis of the purified pFCC-1 product revealed that only one of the two oxygen atoms newly introduced into Pheide a in the course o f the cleavage reaction is derived from molecular oxygen. Analysis of the fragment ions located the oxygen atom derived from molecular oxyge n on the formyl group of pyrrole B, This finding demonstrates that the cleavage of Pheide a in vascular plants is catalyzed by a monooxygena se. Chlorophyll breakdown is therefore indicated to be mechanistically related in higher plants and in the green alga Chlorella protothecoid es.