Chlorophyll catabolism - Structures, mechanisms, conversions

Several chlorophyll catabolites have been isolated and characterised from higher plants (angiosperms) and from a green algae (chlorophyte) during the last five years. A common feature of all chlorophyll catabolites isolated thus far is the unexpected regioselectivity of the oxidative cleavage of the macrocycle at the C(5)-methene bridge, yielding bile-pigment-like 19-formyl-1[21H,23H]bilinones. Recent in vivo O-18,18(2)- labelling experiments performed with the green algae Chlorella protothecoides showed unequivocally that a mono-oxygenase is involved in I the primary oxidative cleavage step of the chlorophyll macrocycle. Moreover, the main red chlorophyll a catabolite from C. protothecoides has been chemically converted into the skeleton of the colourless catabolites isolated from angiosperms. On this basis, a likely catabolic pathway is outlined, which comprises all the products of chlorophyll breakdown isolated until now from both a chlorophyte and from higher plants.