ISSN:
1399-3054
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Biology
Notes:
The possible implication of NADP-dependent malic enzyme (NADP-ME; L-malate:NADP oxidoreductase [oxaloacetate-decarboxylating], EC 1.1.1.40) in fatty acid synthesis was examined in Ricinus communis L. cotyledons, NADP-ME catalyses the conversion of L-malate to pyruvate and NADPH, potential substrates for fatty acid synthesis. NADP-ME activity and protein levels were monitored during germination, up to 20 days postimbibition. The developmental profile showed a peak in activity (6 times with respect to the basal value) and immunoreactive protein (a single 72-kDa band using anti-maize NADP-ME antibodies) around day 7. The enzyme was partially purified (41-fold) and its kinetics characterized. The optimum pH was around 7.1. Km values for L-malate and NADP+ were 0.68 mM and 8.2 μM respectively. The enzyme used Mg2+ or Mn2+ as essential cofactors. Several metabolites were assayed as potential enzyme modulators. Succinate, CoA, acetyl-CoA and palmitoyl-CoA were activators of NADP-ME, at saturating or sub-saturating substrate concentrations, K2 values for CoA and derivative compounds were in the micromolar range (i.e., 0.8 μM for acetyl-CoA). No significant effects were obtained with other Krebs cycle intermediates and amino acids (i.e. 2-oxoglutarate, glutamate, glutamine, fumarate). The activity was 29 times higher in the forward (decarboxylating) direction compared to the reverse direction. These results hint at cotyledon NADP-ME behaving as a regulatory enzyme in R. communis. Its activity is responsive to metabolites of the fatty acid synthesis pathway, and thus a role in this metabolism is suggested.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1399-3054.1997.tb01069.x
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