Substrate Specificity of Chlorophyll(ide) b Reductase in Etioplasts of Barley (Hordeurn Vulgare L.)

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Abstract
Enzyme activity of chlorophyll(ide) b reductase is present in etioplasts. Recently the conversion of chlorophyllide b to chlorophyll a via 71-hydroxychlorophyll a was demonstrated in barley etioplasts. We used zinc pheophorbide b for a detailed investigation of the reduction of the 7-formyl group to the 7l-hydroxy compound in intact barley etioplasts. The reaction proceeded likewise before esterification and after esterification with phytyl diphosphate. The metal-free pheophorbide b, that is not accepted by chlorophyll synthase for esterification, is reduced to 71-hydroxypheophorbide a to a small extent. The zinc (132S)-pheophorbide b is at least equally well accepted for reduction as the epimer with the 132R 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 71-hydroxy compound. The significance of chlorophyll b reduction for acclimation from shade to sun leaves and for chlorophyll degradation is discussed.