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Isoflavones as constituents of plant foods : Isoflavone als Bestandteile pflanzlicher Lebensmittel
(2004)
Vor dem Hintergrund der Diskussion über die ernährungsphysiologische Bedeutung von Isoflavonen befasst sich die vorliegende Übersichtsarbeit auf der Basis von 186 Literaturquellen mit der Struktur, dem Vorkommen, der Aufnahme, der Biosynthese, der Resorption, dem Metabolismus und der biologischen Wirkung dieser Untergruppen der Pflanzenphenole. Diskutiert werden sowohl positive als auch negative biologische Wirkungen dieser Verbindungen. Strukturabhängig können die Isoflavone mit anderen Lebensmittelinhaltsstoffen in Wechselwirkung treten. With the background of the actual ongoing discussion on the nutritional
Soya glycinin was derivatized with different phenolic substances (caffeic-, chlorogenic-, gallic acid and quercetin). The protein derivatives formed have been characterized in terms of their properties where they showed changes in the content of free epsilon-amino groups, tryptophan and thiol groups. The derivatives have also been characterized in terms of their solubility at different pH-values to document the influence on the functional properties. Another objective of this paper was to demonstrate the influence on the digestibility of the proteins with one of the main enzymes of the gastro-intestinal tract (pancreatin) on the basis of in vitro experiments after derivatization with phenolic substances. The enzymatic digestion of the derivatized proteins was promoted.
The application of mass spectrometry for the characterization of food proteins represents one of the most important tools in food chemistry and nutritional science. In the last few years there has been a tremendous development in the classical questions with regard to determination of molecular mass, identification amino acid sequence and structure of proteins. With these technical improvements, it is becoming more and more interesting to characterize the changes involved in proteins embedded in the food matrix as a result of their technological processing, especially in terms of the influence on their functional, nutritional and phsiological properties. Many such posttransational protein modifications occuring due to reactions with other food constituents (e.g. secondary plant metabolites) provide a series of possible fields for application of a sample preparation with a soft ionisation technique using mass spectrometry. The matrix assisted laser desorptions/ionisation ? time of flight ? mass spectrometry (MALDI-TOF-MS) and the surface enhanced laser desorptions/ionisation ? time of flight ? mass spectrometry (SELDI-TOF-MS) have become since than two of the most important methods of choice for solving of such questions and these both techniques have been described here with correponding examples.
During processing and storage, phenolic compounds (PCs) may react with food protein bound amino acids (AAs). Such reactions have been reported to change physicochemical and to decrease in vitro digestion properties of proteins. A rat growth and nitrogen (N) balance study was conducted to prove whether derivatization with chlorogenic acid (CA) affects the nutritional quality of beta-lactoglobulin (beta-LG). Test diets (10% protein level) contained nonderivatized beta-LG (LG, treated under omission of CA), low derivatization level beta-LG (LGL), high derivatization level beta-LG (LGH), or casein supplemented with L-methionine (0.3% of diet; C+met) as an internal standard. An additional group received untreated beta-LG supplemented with pure CA (1.03% of diet; LG+CA). The AA composition of test proteins, plasma AAs, and liver glutathione (GSH) concentrations were determined. Protein digestibility-corrected amino acid score (PDCAAS) was calculated using human or rat AA requirement patterns and rat fecal digestibility values. N excretion was significantly higher in feces and lower in urine of rats fed with LGH as compared to LG and LGL. Consequently, true N digestibility (TND) was significantly lower with LGH as compared to LG and LGL. The lower content of methionine, cysteine, lysine, and tryptophan in LGH corresponded to a reduced TND. Net protein utilization (NPU) was not different between treated beta-LG fed diet groups but was lower than in LG+CA and C+met fed groups. Only at a relatively high level of derivatization with CA, the otherwise good nutritional quality of beta-LG is affected so that TND is reduced, while NPU still remains unaffected. Derivatization of beta-LG with CA does not seem to lead to an additional deficiency in a specific indispensable AA in growing rats fed with 10% protein
Soy glycinin (SG) and soy trypsin inhibitor (STI) were derivatized by chlorogenic- and caffeic acid (cinnamic acids, C6 - C3 - structure), and by gallic acid representing hydroxybenzoic acids (C6 - C1 - structure). Further, the flavonoids, flavone, apigenin, kaempferol, quercetin and myricetin (C6 - C3 - C6 - structure) were also caused to react with soy proteins to estimate the influence of the number and the position of hydroxy substituents. The derivatization caused a reduction of lysine, cysteine and tryptophan residues in the soy proteins. The isoelectric points of the derivatives were shifted to lower pH values and formation of high molecular fractions was documented. The derivatives were characterized in terms of their solubility at different pH-values to document the influence on the functional properties. The structural changes induced were studied using circular dichroism (CD), differential scanning calorimetry (DSC) , intrinsic fluorescence, and binding of anilinonaphthalenesulfonic acid. The influence of derivatization on the in-vitro digestibility with trypsin, chymotrypsin, pepsin and pancreatin was also assessed. The effect on the trypsin inhibitor activity of all the resulting STI derivatives was studied, the latter being reduced.
Assessment of the reactivity of selected isoflavones against proteins in comparison to quercetin
(2004)
Selected isoflavones (genistein, daidzein, formononetin, prunetin, biochanin A and two synthetic isoflavones) were allowed to interact with soy and whey proteins. The reaction products were analyzed in terms of covalent binding at the nucleophilic side chains of proteins. Changes in molecular properties of the proteins derivatives were documented by SDS-PAGE, IEF and SELDI-TOF-MS. The structural changes induced were studied using circular dichroism (CD). The in vitro digestibility was assessed with trypsin. The results show that the occurrence of the catechol moiety, i.e. the two adjacent (ortho) aromatic hydroxyl groups on ring B of the flavonoid structural skeleton appears to be perquisite condition for covalent binding to proteins. The catechol moiety on ring A was less reactive. Its absence lead to a slight or no significant reaction, although non-covalent interactions may still be possible even when lacking this structural element. A comparison of the data is also made with quercetin representing the flavonols.
The intention of this study was to increase the knowledge on the composition and structure of coffee bean proteins and the changes induced in them especially with regard to their interactions with the phenolic compounds also present. For this purpose green coffee beans were extracted by means of standard methanol extraction to quantify the chlorogenic acid content. Different solubilisation buffers were applied to extract the protein fractions with or without prior fat removal. The protein samples thus obtained were analysed by different methods (RP-HPLC, SDS-PAGE and SELDI-TOF- MS). Preliminary model studies were performed to characterize the interactions between the isolated green coffee protein fractions and chlorogenic acid (the major phenolic compound in coffee beans) with the intention of fulfilling the ultimate goal of characterizing such reactions in roasted coffee. The results show that the content of chlorogenic bound covalently to the protein increases. A reaction with the nucleophilic protein side chains (tryptophan, cystein and lysine) was recorded. Cross-inked protein polymers were also detected, whereby the a-chain was found to be more reactive. These reactions effect the solubility of the coffee bean proteins, the latter in turn becoming more acidic in nature. The secondary structure was affected only slightly as determined by circular dichroism. The in-vitro tryptic digestibility was also influenced, where again the cc-chain seems to be more susceptible. The observed polymerisation due to derivatisation by chorogenic acid declines the digestion. Similar digestion behaviour was also observed during tryptic hydrolysis of roasted coffee compared to that of green coffee, roasting allowing more stronger denaturation caused by the accompanying Maillard reaction. The derivatised green coffee bean proteins were found to have moderate antioxidative capacity