Vergleichende Untersuchungen zum thermischen Verhalten von Maisproteinen 2. Mitt. Untersuchungen an Prolamin und Glutelin des Maises

Comparative investigations on thermic reactions of maize proteins. Part 2. Investigations on prolamin and glutelin of maize Zein and Glutelin show in comparison to outgoing values a decrease in protein solubility, most probably due to cross-linking reactions, as proved by SDS-polyacrylamid-gelelectrophoresis investigation. The addition of D-glucose to the reaction-system increases the degree of aggregation due to nonenzymatic browning. This cross-linking reaction takes place in conjunction with decrease in glutamine, lysine and arginine amino acid residues of the investigated proteins.     

Proteinmodifizierung durch Hochdruckhomogenisation

Protein modification by high pressure homogenization During this work different proteins deriving from beans, soy beans and egg white were exposed to a pressure of 1.5. 10⁸ Pa using a high pressure homogenizer. The resulting products were tested with respect to their functional properties (e.g. foam quality, water binding capacity) which were correlated with the molecular data of the proteins. In case of proteins from soy beans and egg white an increased foamability was achieved. For proteins from beans the water binding capacity could be increased. To achieve a better understanding of the processes involved in protein modification during homogenization, further studies are necessary.     

Reactions of chlorogenic acid and quercetin with a soy protein isolate--influence on the in vivo food protein quality in rats

Plant phenolic compounds are known to interact with proteins producing changes in the food (e. g., biological value (BV), color, taste). Therefore, the in vivo relevance, especially, of covalent phenol-protein reactions on protein quality was studied in a rat bioassay. The rats were fed protein derivatives at a 10% protein level. Soy proteins were derivatized with chlorogenic acid and quercetin (derivatization levels: 0.056 and 0.28 mmol phenolic compound/gram protein). Analysis of nitrogen in diets, urine, and fecal samples as well as the distribution of amino acids were determined. Depending on the degree of derivatization, the rats fed with soy-protein derivatives showed an increased excretion of fecal and urinary nitrogen. As a result, true nitrogen digestibility, BV, and net protein utilization were adversely affected. Protein digestibility corrected amino acid score was decreased for lysine, tryptophan, and sulfur containing amino acids.     

Selected physico-chemical properties of succinylated legumin from pea (Pisum sativum L.)

Selected physico-chemical properties of pea legumin before and after succinylation have been investigated using isoelectric focusing, PAGE, SDS-PAGE, hydrophobicity measurements, SE-HPLC and RP-HPLC. Exhaustive succinylation shifted the I.P. of legumin from 4.75 to 3.5. The stepwise dissociation of legumin by increasing succinylation has been confirmed both by means of PAGE in a nondenaturing system, and by SE-HPLC. The results of SDS-PAGE provided evidence for the exposure of α-polypeptide chains in the native legumin. High succinylation resulted in a decrease of the surface hydrophobicity (S₀) measured by both fluorescence probes (cis-parinaric acid and anilino-naphtalene sulfonic acid). RP-HPLC gave a response both to conformational changes and the introduced succinyl residues.     

Interactions of glycinin with plant phenols--influence on chemical properties and proteolytic degradation of the proteins.

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.     

Reactions of a glucosinolate breakdown product (benzyl isothiocyanate) with myoglobin

The interaction of various amounts of benzyl isothiocyanate (benzyl-ITC) with myoglobin is known to lead to the formation of derivatives. These have been characterised by the determination of solubility, free amino group, tryptophan content and chromatographic as well as electrophoretic behaviour. In the range between 2.5 and 125 mg benzyl-ITC/g protein, all properties of the reaction products correlate with the concentration of benzyl-ITC. However, at 250 mg benzyl-ITC/g myoglobin, a rather unexpected low degree of derivatization, as well as atypical chromatographic and electrophoretic behaviour, is observed. The proposed explanation was that conformational changes in the presence of a high concentration of hydrophobic benzyl-ITC made fewer amino groups accessible to the reagent. To test this hypothesis we have run the reaction under denaturing conditions. The results showed that the reaction of myoglobin with high concentrations of benzyl-ITC in the presence of 8 M urea led to a higher degree of derivatization than in the presence of water only. In addition, the Mr distribution of the reaction products was determined by MALDI-TOF-mass spectrometry and the overall degree of derivatization calculated from the spectra.     

Microwave digestion of proteins

 The time needed for a conventional acidic digestion of peptide bonds in proteins with 6 N HCl was reduced from 18–24 h to 2 h when microwaves were applied. The rate-determining step of the splitting of the peptide bonds depends on the amino acid composition of the proteins. Myoglobin, lysozyme and bovine serum albumin were tested as pure model proteins. The amino acid contents determined after ion-exchange HPLC and postcolumn derivatisation with ninhydrin correlated well with values obtained on the basis of known amino acid sequences. Tryptophan, methionine as well as cysteine/cystine were partly or completely destroyed under the hydrolysis conditions applied. Almost 95% of the thermolabile amino acids (based on theoretical values) could be recovered. The conditions were further applied for the hydrolysis of food proteins like casein, commercially available whey proteins as well as pea proteins, and a comparison with the results of the conventional hydrolysis (24 h) was made. Received: 26 February 1998 / Revised version: 20 March 1998     

Microwave digestion of proteins

 The time needed for a conventional acidic digestion of peptide bonds in proteins with 6 N HCl was reduced from 18–24 h to 2 h when microwaves were applied. The rate-determining step of the splitting of the peptide bonds depends on the amino acid composition of the proteins. Myoglobin, lysozyme and bovine serum albumin were tested as pure model proteins. The amino acid contents determined after ion-exchange HPLC and postcolumn derivatisation with ninhydrin correlated well with values obtained on the basis of known amino acid sequences. Tryptophan, methionine as well as cysteine/cystine were partly or completely destroyed under the hydrolysis conditions applied. Almost 95% of the thermolabile amino acids (based on theoretical values) could be recovered. The conditions were further applied for the hydrolysis of food proteins like casein, commercially available whey proteins as well as pea proteins, and a comparison with the results of the conventional hydrolysis (24 h) was made. Received: 26 February 1998 / Revised version: 20 March 1998