Human Sensory, Taste Receptor, and Quantitation Studies on Kaempferol Glycosides Derived from Rapeseed/Canola Protein Isolates.

Beyond the key bitter compound kaempferol 3-O-(2‴-O-sinapoyl-ß-d-sophoroside) previously described in the literature (1), eight further bitter and astringent-tasting kaempferol glucosides (2-9) have been identified in rapeseed protein isolates (Brassica napus L.). The bitterness and astringency of these taste-active substances have been described with taste threshold concentrations ranging from 3.3 to 531.7 and 0.3 to 66.4 µmol/L, respectively, as determined by human sensory experiments. In this study, the impact of 1 and kaempferol 3-O-ß-d-glucopyranoside (8) on TAS2R-linked proton secretion by HGT-1 cells was analyzed by quantification of the intracellular proton index. mRNA levels of bitter receptors TAS2R3, 4, 5, 13, 30, 31, 39, 40, 43, 45, 46, 50 and TAS2R8 were increased after treatment with compounds 1 and 8. Using quantitative UHPLC-MS/MSMRM measurements, the concentrations of 1-9 were determined in rapeseed/canola seeds and their corresponding protein isolates. Depending on the sample material, compounds 1, 3, and 5-9 exceeded dose over threshold (DoT) factors above one for both bitterness and astringency in selected protein isolates. In addition, an increase in the key bitter compound 1 during industrial protein production (apart from enrichment) was observed, allowing the identification of the potential precursor of 1 to be kaempferol 3-O-(2‴-O-sinapoyl-ß-d-sophoroside)-7-O-ß-d-glucopyranoside (3). These results may contribute to the production of less bitter and astringent rapeseed protein isolates through the optimization of breeding and postharvest downstream processing.

Channeling of eukaryotic diacylglycerol into the biosynthesis of plastidial phosphatidylglycerol.

Plastidial glycolipids contain diacylglycerol (DAG) moieties, which are either synthesized in the plastids (prokaryotic lipids) or originate in the extraplastidial compartment (eukaryotic lipids) necessitating their transfer into plastids. In contrast, the only phospholipid in plastids, phosphatidylglycerol (PG), contains exclusively prokaryotic DAG backbones. PG contributes in several ways to the functions of chloroplasts, but it is not known to what extent its prokaryotic nature is required to fulfill these tasks. As a first step toward answering this question, we produced transgenic tobacco plants that contain eukaryotic PG in thylakoids. This was achieved by targeting a bacterial DAG kinase into chloroplasts in which the heterologous enzyme was also incorporated into the envelope fraction. From lipid analysis we conclude that the DAG kinase phosphorylated eukaryotic DAG forming phosphatidic acid, which was converted into PG. This resulted in PG with 2-3 times more eukaryotic than prokaryotic DAG backbones. In the newly formed PG the unique Delta3-trans-double bond, normally confined to 3-trans-hexadecenoic acid, was also found in sn-2-bound cis-unsaturated C18 fatty acids. In addition, a lipidomics technique allowed the characterization of phosphatidic acid, which is assumed to be derived from eukaryotic DAG precursors in the chloroplasts of the transgenic plants. The differences in lipid composition had only minor effects on measured functions of the photosynthetic apparatus, whereas the most obvious phenotype was a significant reduction in growth.

Two long-chain acyl-CoA synthetases from Arabidopsis thaliana involved in peroxisomal fatty acid beta-oxidation.

Post-germinative growth of oilseeds is dependent on the breakdown of the stored lipid reserves. Long-chain acyl-CoA synthetase activities (LACS) are critically involved in this process by activating the released free fatty acids and thus feeding the beta-oxidation cycle in glyoxysomes. Here we report on the identification of two LACS genes, AtLACS6 and AtLACS7 from Arabidopsis thaliana coding for peroxisomal LACS proteins. The subcellular localization was verified by co-expression studies of spectral variants of the green fluorescent protein (GFP). While AtLACS6 is targeted by a type 2 (PTS2) peroxisomal targeting sequence, for AtLACS7 a functional PTS1 as well as a PTS2 could be demonstrated. Possible explanations for this potentially redundant targeting information will be discussed. Expression studies of both genes revealed a strong induction 1 day after germination resembling the expression pattern of other genes involved in beta-oxidation. Analysis of the substrate specificities of the two LACS proteins demonstrated enzymatic activity for both enzymes with the whole spectrum of fatty acids found in stored lipid reserves. These results suggest that both LACS proteins might have overlapping functions and are able to initiate beta-oxidation in plant peroxisomes.