Biosynthesis of protochlorophyllide a from Mg-protoporphyrin IX in vitro

Using a column chromatographic analytical procedure the conversion of 3H-labeled Mg-protoporphyrin IX to 3H-labeled protochlorophyllide a in crude homogenates of etiolated wheat seedlings was demonstrated. Optimum rates of (3H-labeled) protochlorophyllide a biosynthesis from (3H-labeled) Mg-protoporphyrin IX in vitro were observed when reaction mixtures were incubated at 37 degC (pH 7.8) and were 40 μm in Mg-protoporphyrin IX. The biosynthetic activity of such reaction mixtures was not significantly depressed when incubation was carried out at room temperature (pH 7.8). The results of further studies of this biosynthetic sequence in vitro suggested that S-adenosyl-l-methionine, NAD+, phosphate, coenzyme A and ATP are required. Protochlorophyllide a biosynthesis from Mg-protoporphyrin IX in vitro is discussed in terms of a multienzyme complex as possibly being involved in the biosynthetic conversion of Mg-protoporphyrin IX monomethyl ester (or its 4-ethyl-(4-desvinyl)-derivative) to protochlorophyllide a in plants. (author)