JXB Advance Access originally published online on December 20, 2004
Journal of Experimental Botany 2005 56(412):577-585; doi:10.1093/jxb/eri046
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RESEARCH PAPER |
Fatty acid synthesis and the oxidative pentose phosphate pathway in developing embryos of oilseed rape (Brassica napus L.)
1School of Biological Sciences, University of Manchester, 3.614 Stopford Building, Oxford Street, Manchester M13 9PT, UK
2Department of Metabolic Biology, John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, UK
* To whom correspondence should be addressed. Fax: +44 (0)1603 450014. E-mail: steve.rawsthorne{at}bbsrc.ac.uk
The potential role of the plastidial oxidative pentose phosphate pathway (OPPP) in providing the NADPH for fatty acid synthesis in plastids from developing embryos of Brassica napus (L.) has been investigated. Measurements of distributions of enzyme activities in fractions obtained from homogenates of isolated embryos have revealed that the glucose 6-phosphate and 6-phosphogluconate dehydrogenases are present in both cytosol and plastid, as is ribose 5-phosphate isomerase. However, transketolase and transaldolase are most probably confined to the plastid, while ribulose 5-phosphate epimerase is essentially cytosolic, although a very small proportion of plastid-localized activity cannot be ruled out. The activity of the OPPP in intact plastids was measured by the release of 14CO2 from [1-14C]glucose 6-phosphate. Activity was detectable in the absence of electron sinks created by the addition of metabolites to the incubation media and was stimulated 1.3-, 3.2-, and 7.9-fold by the respective additions of glutamine plus 2-oxoglutarate, cofactors and substrates for fatty acid synthesis, or methyl viologen. An increase in OPPP activity in response to additions that are absolutely required for fatty acid synthesis in these isolated plastids provides direct evidence that these two processes are connected, most probably by NADP/NADPH metabolism. The OPPP activity with methyl viologen was more than twice that during fatty acid synthesis, suggesting that the latter is not limited by OPPP capacity. Light energy may also contribute to reductant provision and, consistent with the possibility of maintenance of a balance of NADPH from light and the OPPP, glucose 6-phosphate dehydrogenase activity in the isolated plastids was decreased by light or by DTT.
Key words: Brassica napus, embryo, fatty acid synthesis, oxidative pentose phosphate pathway, plastid
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