Chloroplast biogenesis: detection of a magnesium protoporphyrin diester pool in plants

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Abstract
The discovery of a novel metalloporphyrin pool in etiolated cucumber [Cucumis sativus L. cv. Beit Alpha MR] cotyledons and in dark-grown Euglena gracilis is described. The novel pool exhibited chromatographic properties of a fully esterified metalloporphyrin, devoid of free carboxylic groups, and the spectrophotometric and spectrofluorometric properties of a Mg protoporphyrin. Demetalation and hydrolysis indicated that the tetrapyrrole moiety of the metalloporphyrin was a protoporphyrin diester. High-pressure liquid chromatography of the fully esterified metalloporphyrin pool and gas chromatographic/mass spectroscopic analysis of the saponified alcohol fraction revealed that the latter was made up of 3 major long-chain alcohols. None of those alcohols was identifiable, however, with known isoprenoids such as geraniol, farnesol or phytol. Similar analysis of the saponified alcohol fraction of the protochlorophyllide ester pool likewise revealed the presence of 3 major long-chain alcohols none of which was identifiable with known isoprenoid alcohols or with the alcohols of the novel metalloporphyrin pool. On the basis of the above observations, the novel metalloporphyrin pool was tentatively identified as a Mg protoporphyrin diester pool. Apparently, this pool is a metabolic intermediate of the fully esterified branch of the chlorophyll biosynthetic pathway and is probably the precursor of the protochlorophyllide ester pool in plants.

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