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

The discovery of a novel metalloporphyrin pool in etiolated cucumber cotyledons and in dark-grown Euglena gracilis is described. The novel pool exhibited the chromatographic properties of a fully esterified metalloporphyrin, devoid of free carboxylic groups, and the spectrophotometric and spectrofluorometric properties of a magnesium 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 three 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 three 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 magnesium protoporphyrin diester pool. It is suggested that this pool is a metabolic intermediate of the fully esterified branch of the chlorophyll biosynthetic pathway [Rebeiz, C. A., Smith, B. B., Matthesis, I. R., Cohen, C. E., & McCarthy, S. A. (1978) in Chloroplast Development (Akoyunoglou, G., & Argyroudi-Akoyunoglou, J. H., Eds.) pp 56-76, Elsevier/North-Holland Bio-Medical Press, Amsterdam] and is probably the precursor of the protochlorophyllide ester pool in plants.