Absolute Configuration of Protochlorophyllideaand Substrate Specificity of NADPH−Protochlorophyllide Oxidoreductase

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Protochlorophyllide a was isolated from dark-grown barley ( Hordeum vulgare L.) seedlings. The circular dichroism spectrum was identical with that of protochlorophyll a prepared from chlorophyll a by dehydrogenation. The mirror image was obtained for the circular dichroism spectrum of protochlorophyllide a ‘ that had been prepared from chlorophyllide a ‘. These results prove the 13 2 ( R ) configuration of natural protochlorophyllide a . Incubation of the seedlings with 5-aminolevulinate resulted in accumulation of protochlorophyllide a . This proved to be partially racemized already within the plants. Circular dichroism spectra were also obtained for 13 2 ( R )-methoxy and 13 2 ( S )-methoxy derivatives of zinc protopheophorbide a that cannot racemize. Using the increment of the methoxy group by comparison with the respective methoxypyro compound, the circular dichroism spectrum of zinc protopheophorbide a was calculated. Kinetics of racemization indicated a faster reaction for zinc than for magnesium complexes. The enzyme NADPH−protochlorophyllide oxidoreductase solubilized from isolated prolamellar bodies of dark-grown wheat ( Triticum aestivum L.) does not accept protochlorophyllide a ‘ or any compound with substituents at C-13 2 different from protochlorophyllide a . The substrate specificity is compared with that of chlorophyll synthase.