Terpenoid Metabolism in Plastids

Abstract

The synthesis of a-tocopherol from 2,3-dimethylphytylquinol and Sadenosyl-L-methionine was achieved using Capsicum amwuan fruit chromoplasts. The enzymes involved in the cyclization (2,3-dimethyl-phytylquinol cyclase) and methylation (S-adenosyl methionine:y-tocopherol methyltransferase) are both localized in the chromoplast membrane fraction (envelopes and/or a-chlorophyll lamellae), in contrast to the stroma fraction.Terpenoid molecules play important structural and functional roles during plastid membrane organization (for a review, see Refs. 8 and 9). Among these molecules, the biosynthesis of atocopherol has been studied recently in isolated plastids (2,3,13,15). Based on the observed reactions, the following pathway has been proposed for the terminal steps (14, 17) 2,3-Dimethylphytylquinol -yT + SAMa T cyclization methylation However, in chloroplast subfractions, the cyclization reaction and the subsequent methylation of the endogenously preformed yT2 which yield aT have not been observed, although exogenously supplied yT is converted to aT by the plastid envelopes (13). Using Capsicum chromoplasts, we show that the plastid membranes (envelopes and a-Chl lamellae) are the sites of 2,3-dimethylphytylquinol cyclase and SAM:y-tocopherol methyltransferase activities which yield caT from dimethylphytylquinol. MATERIALS AND METHODSCapsicum annuum fruit chloroplasts were prepared according to a modified procedure (12, 18). Details will be given elsewhere. Capsicum chromoplasts were isolated from 2.5 kg of fruits according to a previously described method (3). The intact chromoplasts recovered at the 0.84 to 1.45 M interface of the gradient system were diluted (I ml suspension + I ml 50 mm Tris-HCI, pH 7.6, containing 10 ml DTT) and centrifuged at 4000g for 10 min. The pellet was homogenized in a loosely fitting Potter-Elvehjem homogenizer with 50 ml of a medium containing 10 mm DTT, 50 mm Tris-HCl (pH 7.6). The suspension was kept at 0 to 4°C for 10 min and centrifuged at 170,000g (R,,.) for 3.3 h in a precooled Beckman L3-50 equipped with a 42.1 rotor. At the completion, ' Supported by grants from the Mission de la Recherche and from the Centre National de la Recherche Scientifique (AI 03.1269).2 Abbreviations: yT, y-tocopherol; aT, a-tocopherol; SAM, S-adenosyl-L-methionine. the stroma was separated from the plastid membranes (chromoplast envelope and a-Chl lamellae) as revealed by the exclusive presence of galactosyltransferase activities in the chromoplast membranes and the phytoene synthetase activities in the stroma (4).The assay for a-tocopherol synthesis was based on the cyclization and methylation of 2,3-dimethylphytylquinol in the presence of S-adenosyl-L-[methyl-'4C]methionine (Amersham, France).

Molecular aspects of plastid envelope biochemistry

“…The same pathway has been demonstrated in pepper (Capsicum annuurn) chromoplasts [127]. The other major chloroplast prenylquinone, plastoquinone-9, is also synthesized in the inner envelope membrane by condensation of homogentisic acid and solanesyl diphosphate to form 2-methyl-6-solanesylquinol, which is methylated and oxidized to form successively plastoquinol-9 and plastoquinone-9 [ 1221.…”

Salinity effect on nutritional value, chemical composition and bioactive compounds content of Cichorium spinosum L.

“…According to Camara, Bardat, Seye, D'Harlinge, and Moneger (1982), a-tocopherol and chlorophyll are both located in chloroplasts and a-tocopherol acts as protecting agent of chlorophyll, therefore chlorophyll loss could be the result of a-tocopherol decrease and consequently the inactivation of the involved protection mechanisms.…”

Biosynthesis of Thylakoid Membrane Lipids

“…Tocopherols are synthesized by condensation of homogentisic acid and a pyrophosphate to form 2-methyl-6-prenylquinol (Soll et al, 1980a;Fiedler et al, 1982).By a series of methylations and cyclization, the 2-methyl-6-prenylquinol gives rise successively to 2,3-dimethyl-6-prenylquinol, or and finally or tocotrienol (Soll et al, 1980a). The same pathway has been demonstrated in pepper (Capsicum annuum) chromoplasts (Camara et al, 1982). The other major chloroplast prenylquinone, plastoquinone-9, is also synthesized in the inner envelope membrane by condensation of homogentisic acid and solanesylpyrophosphate to form 2-methyl-6-solanesylquinol, which is methylated and oxidized to form successively plastoquinol-9 and plastoquinone-9 (Soll et al, 1985).…”