title: Fatty acid synthesis and the oxidative pentose phosphate pathway in developing embryos of oilseed rape (Brassica napus L.).
abstract: 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 (14)CO(2) from [1-(14)C]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.
authors.affiliation: School of Biological Sciences, University of Manchester, 3.614 Stopford Building, Oxford Street, Manchester M13 9PT, UK.
authors.forename: David
authors.initials: D
authors.lastname: Hutchings
authors.forename: Stephen
authors.initials: S
authors.lastname: Rawsthorne
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Emes
mesh_terms: D029688:Brassica napus / Q000196:embryology / Q000201:enzymology*; D002245:Carbon Dioxide; D005227:Fatty Acids / Q000096:biosynthesis*; D019298:Glucose-6-Phosphate; D005954:Glucosephosphate Dehydrogenase / Q000378:metabolism; D010084:Oxidation-Reduction; D010085:Oxidative Phosphorylation; D010427:Pentose Phosphate Pathway; D018087:Plastids / Q000201:enzymology; D012639:Seeds / Q000201:enzymology*; D013997:Time Factors
chemical_list: D005227:Fatty Acids; D002245:Carbon Dioxide; D019298:Glucose-6-Phosphate; D005954:Glucosephosphate Dehydrogenase