SUBSTRATE-SPECIFICITY OF CHLOROPHYLL(IDE)-B REDUCTASE IN ETIOPLASTS OF BARLEY (HORDEUM-VULGARE L)

Enzyme activity of chlorophyll(ide) b reductase is present in etioplas ts. Recently the conversion of chlorophyllide b to chlorophyll a via 7 (1)-hydroxychlorophyll a was demonstrated in barley etioplasts. We use d zinc pheophorbide b for a detailed investigation of the reduction of the 7-formyl group to the 7'-hydroxy compound in intact barley etiopl asts. The reaction proceeded likewise before esterification and after esterification with phytyl diphosphate. The metal-free pheophorbide b, that is not accepted by chlorophyl synthase for esterification, is re duced to 7(1)-hydroxypheophorbide a to a small extent. The zinc (13(2) S)-pheophorbide b is at least equally well accepted for reduction as t he epimer with the 13(2)R configuration of natural chlorophyll b. The reaction requires NADPH or NADH, although the latter is less effective . ATP is not required for the first step to the 7(1)-hydroxy compound. The significance of chlorophyll b reduction for acclimation from shad e to sun leaves and for chlorophyll degradation is discussed.