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M A van Boekel

Publications and source records attributed to M A van Boekel.

At least 19 recordsLinked to original sources
PubMed2001

Kinetic aspects of the Maillard reaction: a critical review.

The literature concerning the kinetics of the Maillard reaction was critically discussed according to the initial, intermediate and advanced stages, as this is the way the Maillard reaction is traditionally analysed. For each stage, a division is made between simple kinetics and complex kinetics. Simple kinetics means that the general rate law is used and results are reported as zero-, first- or second-order reactions (sometimes a fractional order). It is emphasized that this approach for a complex reaction as the Maillard reaction only results in a mathematical fit procedure, not in mechanistic insight. The rate constants and activation energies derived are in fact composed of several elementary rate constants. With complex kinetics, i.e. trying to establish the kinetics for individual reaction steps, more mechanistic information can be extracted. However, there are conflicting results in literature and the interpretation is not always correct, as is shown in several examples. A summary of activation energies reported for the various stages in the Maillard reaction reveals large discrepancies, probably reflecting strong effects of experimental conditions on results that are obtained. Careful control of experimental conditions and proper kinetic analysis of the various stages in the Maillard reaction should lead to more consistent results in the future.

Carbohydrate Metabolism↗
PubMed2001

Evolution of the aldose reductase-related gecko eye lens protein rhoB-crystallin: a sheep in wolf's clothing.

rhoB-crystallin (AJ245805) is a major protein component (20%) in the eye lens of the gecko Lepidodactylus lugubris. Limited peptide sequence analysis earlier revealed that it belongs to the aldo-keto reductase superfamily, as does the frog lens rho-crystallin. We have now determined the complete cDNA sequence of rhoB-crystallin and established that it is more closely related to the aldose reductase branch of the superfamily than to frog rho-crystallin. These gecko and frog lens proteins have thus independently been recruited from the same enzyme superfamily. Aldose reductase is implicated in the development of diabetic cataract in mammals, and, if active, rhoB-crystallin might be a potential risk for the gecko lens. Apart from a replacement 298 Cys --> Tyr, rhoB-crystallin possesses all amino acid residues thought to be required for catalytic activity of the aldose reductases. However, modeling studies of the rhoB-crystallin structure indicate that substrate specificity and nicotinamide cofactor affinity might be affected. Indeed, neither recombinant rhoB-crystallin nor the reverse mutant 298 Tyr --> Cys showed noticeable activity toward aliphatic and aromatic substrates, although cofactor binding was retained. Various other oxidoreductases are known to be recruited as abundant lens proteins in many vertebrate species; rhoB-crystallin demonstrates that an aldose reductase-related enzyme also can be modified to this end.

Aldehyde Reductase↗
PubMed2001

Reactions of monosaccharides during heating of sugar-casein systems: building of a reaction network model.

The Maillard reaction is important during the heating and processing of foods for its contribution to food quality. To control a reaction as complex as the Maillard reaction, it is necessary to study the reactions of interest quantitatively. In this paper the main reaction products in monosaccharide-casein systems, which were heated at 120 degrees C and pH 6.7, were identified and quantified, and the reaction pathways were established. The main reaction routes were (i) sugar isomerization, (ii) degradation of the sugar into carboxylic acids, and (iii) the Maillard reaction itself, in which not only the sugar itself but also its reaction products react with the epsilon-amino group of lysine residues of the protein. Significant differences in reaction mechanism between aldose and ketose sugars were observed. Ketoses seemed to be more reactive in the sugar degradation reactions than their aldose isomers, and whereas the Amadori product was detected as a Maillard reaction intermediate in the aldose-casein system, no such intermediate could be found in the ketose-casein system. The reaction pathways found were put together into a model, which will be evaluated by kinetic modeling in a subsequent paper.

Carbohydrates↗
PubMed2001

Effects of pH and heat treatments on the structure and solubility of potato proteins in different preparations.

The soluble potato proteins are mainly composed of patatin and protease inhibitors. Using DSC and both far-UV and near-UV CD spectroscopy, it was shown that potato proteins unfold between 55 and 75 degrees C. Increasing the ionic strength from 15 to 200 mM generally caused an increase in denaturation temperature. It was concluded that either the dimeric protein patatin unfolds in its monomeric state or its monomers are loosely associated and unfold independently. Thermal unfolding of the protease inhibitors was correlated with a decrease in protease inhibitor activities and resulted in an ionic strength dependent loss of protein solubility. Potato proteins were soluble at neutral and strongly acidic pH values. The tertiary structure of patatin was irreversibly altered by precipitation at pH 5. At mildly acidic pH the overall potato protein solubility was dependent on ionic strength and the presence of unfolded patatin.

Calorimetry, Differential Scanning↗
PubMed2001

Chaperone function of mutant versions of alpha A- and alpha B-crystallin prepared to pinpoint chaperone binding sites.

A major stress protein, alpha-crystallin, functions as a chaperone. Site-directed mutagenesis has been used to identify regions of the protein necessary for chaperone function. In this work we have taken some of the previously described mutants produced and assessed their chaperone function by both a traditional heat-induced aggregation method at elevated temperature and using enzyme methods at 37 degrees C. In general the different assays gave parallel results indicating that the same property is being measured. Discrepancies were explicable by the heat lability of some mutants. Most mutants had full chaperone function showing the robust nature of alpha-crystallin. A mutant corresponding to a minor component of rodent alpha A-crystallin, alpha Ains-crystallin, had decreased chaperone function. Decreased chaperone function was also found for human Ser139--> Arg, Thr144-->Arg, Ser59-->Ala mutants of alpha B-crystallin and double mutants Ser45-->Ala/Ser59-->Ala, Lys103--> Leu/His104-->Ile, and Glu110-->His/His111-->Glu. A mutant Phe27-->Arg that was the subject of previous controversy was shown to be fully active at physiological temperatures.

Amino Acid Sequence↗
PubMed2000

Mutagenicity of heated sugar-casein systems: effect of the Maillard reaction.

The formation of mutagens after the heating of sugar-casein model systems at 120 degrees C was examined by the Ames test, using Salmonella typhimurium strain TA100. Several sugars (glucose, fructose, galactose, tagatose, lactose, and lactulose) were compared in their mutagenicities. Mutagenicity could be fully ascribed to Maillard reaction products and strongly varied with the kind of sugar. The differences in mutagenicity among the sugar-casein systems were caused by a difference in reaction rate and a difference in reaction mechanism. Sugars with a comparable reaction mechanism (glucose and galactose) showed a higher mutagenic activity corresponding with a higher Maillard reactivity. Disaccharides showed no mutagenic activity (lactose) or a lower mutagenic activity (lactulose) than their corresponding monosaccharides. Ketose sugars (fructose and tagatose) showed a remarkably higher mutagenicity compared with their aldose isomers (glucose and galactose), which was due to a difference in reaction mechanism.

Caseins↗
PubMed2000

Effect of UV-A light on the chaperone-like properties of young and old lens alpha-crystallin.

PURPOSE: To study the damaging effect of UV-A irradiation on the chaperone-like properties of alpha-crystallin and the subsequent recovery process of young and old bovine lenses. METHODS: Young and old bovine lenses were kept in organ culture. After 24 hours of incubation they were irradiated with UV-A at 365 nm, and optical quality measurements were performed during the experiments (192 hours). alpha-Crystallin and alpha1-, alphaA2-, alphaB1-, and alphaB2-crystallin subunits were analyzed, separated by gel filtration and cation exchange chromatography, respectively, after different culture times. Protein patterns were obtained after two-dimensional (2-D) gel electrophoresis. Chaperone-like activity was determined on the basis of insulin B-chain and betaL-crystallin aggregation assays. Aggregation of alpha-crystallin was analyzed, tryptophan fluorescence measurements were performed, and alpha-crystallin mRNA levels were determined. RESULTS: The water-soluble alpha-crystallin obtained from old lenses compared with young lenses after UV irradiation had decreased chaperone activity, a higher molecular weight, and increased loss of tryptophan fluorescence. Moreover, alpha-crystallin mRNA virtually disappeared, whereas extra spots on the 2-D protein pattern appeared, possibly because of deamidation. CONCLUSIONS: alpha-Crystallin obtained from old lenses is more affected by irradiation than alpha-crystallin derived from young lenses. Moreover, it appeared that alphaB-crystallin from UV-treated old lenses compared with control lenses was less susceptible to UV-A than alphaA-crystallin. It may well be that alphaB-crystallin protects alphaA-crystallin in vivo.

Aging↗
PubMed1999

Eye lens alphaA- and alphaB-crystallin: complex stability versus chaperone-like activity.

The major lens protein alpha-crystallin is composed of two related types of subunits, alphaA- and alphaB-crystallin, of which the former is essentially lens-restricted, while the latter also occurs in various other tissues. With regard to their respective chaperone capacities, it has been reported that homomultimeric alphaA-crystallin complexes perform better in preventing thermal aggregation of proteins, while alphaB-crystallin complexes protect more efficiently against reduction-induced aggregation of proteins. Here, we demonstrate that this seeming discrepancy is solved when the reduction assay is performed at increasing temperatures: above 50 degrees C alphaA- performs better than alphaB-crystallin also in this assay. This inversion in protective capacity might relate to the greater resistance of alphaA-crystallin to heat denaturation. Infrared spectroscopy, however, revealed that this is not due to a higher thermostability of alphaA-crystallin's secondary structure. Also the accessible hydrophobic surfaces do not account for the chaperoning differences of alphaA- and alphaB-crystallin, since regardless of the experimental temperature alphaB-crystallin displays a higher hydrophobicity. It is argued that the greater complex stability of alphaA-crystallin, as evident upon urea denaturation, and the higher chaperone capacity of alphaB-crystallin at physiological temperatures reflect the evolutionary compromise to obtain an optimal functioning of heteromeric alpha-crystallin as a lens protein.

Anilino Naphthalenesulfonates↗
PubMed1999

Kinetics of thermal inactivation of the extracellular proteinase from Pseudomonas fluorescens 22F: influence of pH, calcium, and protein.

The influence of pH, calcium ion activity, protein, and enzyme purification on the kinetics of heat inactivation of the extracellular proteinase from Pseudomonas fluorescens 22F was studied in the temperature range 80-120 degrees C. At pH 5.5-8.6 the rate of inactivation increased slightly with increasing pH values. The pH dependence of inactivation suggests that the inactivation mechanism is mainly through deamidation. Calcium ion activity had no influence on the kinetics of heat inactivation of the proteinase. Addition of 1.8% sodium caseinate to the enzyme solution slightly decreased the heat stability of the proteinase, possibly because part of the inactivation of the proteinase is caused by aggregation to casein. Purification of the proteinase did not change the rate of thermal inactivation.

Calcium Chloride↗
PubMed1999

Isolation and characterization of patatin isoforms.

Patatin has, so far, been considered a homogeneous group of proteins. A comparison of the isoforms in terms of structural properties or stability has not been reported. A method to obtain various isoform fractions as well as a comparison of the physicochemical properties of these pools is presented. Patatin could be separated in four isoform pools, denoted A, B, C, and D, representing 62%, 26%, 5%, and 7% of the total amount of patatin, respectively. These isoforms differed in surface charge, resulting in a different behavior on anion exchange chromatography, isoelectric focusing, native polyacrylamide gel, and capillary electrophoresis. All isoforms of the patatin family contained proteins with two molecular masses of approximately 40.3 and 41.6 kDa, respectively. The size of this difference in the molar mass (1300 Da) is on the order of one carbohydrate moiety. Despite the biochemical differences given above, no variations in the structural properties nor in the thermal conformational stability could be observed using far-ultraviolet circular dichroism, infrared, and fluorescence spectroscopy.

Carboxylic Ester Hydrolases↗
PubMed1999

Kinetic modeling of the thermal aggregation of patatin.

A kinetic model of the thermal aggregation of patatin is presented based on chromatographic analysis of the proportions of nonaggregated and aggregated patatin. It was observed that the decrease of the amount of nonaggregated patatin proceeded initially quickly and was followed by slower aggregation at longer incubation times. It was shown that this behavior was not due to heterogeneity of the starting material. It was noted that overestimation of the amount of native molecules after a heat treatment, caused by refolding of the unfolded protein during the cooling step prior to the analysis, was significant and could not be neglected. Hence, corrections based on information on the structural properties of patatin were applied. Taking this into account, a model was proposed consisting of a first-order formation of reactive particles, followed by a second-order aggregation reaction. This model described the thermal aggregation of patatin rather accurately and was confirmed by experiments at various protein concentrations.

Carboxylic Ester Hydrolases↗
PubMed1998

Mechanism and kinetics of inactivation at 40-70 degrees C of the extracellular proteinase from Pseudomonas fluorescens 22F.

HPLC size exclusion chromatography experiments showed that during inactivation at 40-70 degrees C of the extracellular proteinase from Pseudomonas fluorescens 22F small molecular mass fragments were formed, indicating that autoproteolysis was at least one of the major causes of inactivation. The formation of small molecular mass fragments and the reaction order indicated that intermolecular autoproteolysis was more likely than intramolecular autodigestion. This was confirmed by computer simulations. The rate constants and the activation enthalpy (delta H++) and entropy (delta S++) for the reactions of the intermolecular autoproteolysis model were derived from computer simulations. delta H++ and delta S++ of the unfolding reaction were 504 kJ mol-1 and 1252 J mol K-1 respectively. delta H++ and delta S++ of the refolding reaction were strongly temperature dependent. The estimates for the enthalpy (delta H0) and entropy (delta S0) difference between the folded and unfolded state as derived from the reaction rate constants of unfolding and refolding were subject to large deviations, owing to accumulation of errors in the estimation of the kinetic characteristics.

Animals↗
PubMed1997

Product quality and food processing: how to quantify the healthiness of a product.

This paper discusses possibilities to measure healthiness of a food product. It is argued that besides measurement of concentration of biologically active compounds it is also necessary to measure biological activity of such compounds in a food. In doing so, it becomes possible to substantiate health claims associated with a food, as well as to optimize food processing with respect to healthiness. Two examples are given of measuring bioactivity in a food, antioxidant activity of apple juice, and (anti)mutagenicity of heated milk. It is also stressed that optimization with respect to healthiness should be done taking other quality aspects (such as organoleptic properties, safety and nutritional value) into account.

Antimutagenic Agents↗
PubMed1997

Antimutagenic activity of casein against MNNG in the E. coli DNA repair host-mediated assay.

The effect of caseinate and soy protein in the diet on the mutagenicity induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was assessed in-vivo and ex-vivo in the DNA-repair host-mediated assay and liquid suspension assay, respectively. Of the two proteins only casein showed a strong antimutagenic activity over the whole digestive tract, except in the stomach. It is suggested that the molecular structure of a protein determines its protective effect against mutagens: casein lacks secondary and tertiary structure so that amino acids are more readily available for interaction with the mutagen than with the amino acids in soy protein which is a globular protein.

Animals↗
PubMed1996

Immobilization of the C-terminal extension of bovine alphaA-crystallin reduces chaperone-like activity.

alpha-Crystallins occur as multimeric complexes, which are able to suppress precipitation of unfolding proteins. Although the mechanism of this chaperone-like activity is unknown, the affinity of alpha-crystallin for aggregation-prone proteins is probably based on hydrophobic interactions. alpha-Crystallins expose a considerable hydrophobic surface to solution, but nevertheless they are very stable and highly soluble. An explanation for this paradox may be that alpha-crystallin subunits have a polar and unstructured C-terminal extension that functions as a sort of solubilizer. In this paper we have described five alphaA-crystallins in which charged and hydrophobic residues were inserted in the C-terminal extension. Introduction of lysine, arginine, and aspartate does not substantially influence chaperone-like activity. In contrast, introduction of a hydrophobic tryptophan greatly diminishes functional activity. CD experiments indicate that this mutant has a normal secondary structure and fluorescence measurements show that the inserted tryptophan is located in a polar environment. However, NMR spectroscopy clearly demonstrates that the presence of the tryptophan residue dramatically reduces the flexibility of the C-terminal extension. Furthermore, the introduction of this tryptophan results in a considerably decreased thermostability of the protein. We conclude that changing the polarity of the C-terminal extension of alphaA-crystallin by insertion of a highly hydrophobic residue can seriously disturb structural and functional integrity.

Animals↗
PubMed1996

The influence of some post-translational modifications on the chaperone-like activity of alpha-crystallin.

We investigated the influence of phosphorylation, glycation, carbamylation and oxidative modification on the capacity of alpha-crystallin to protect beta-crystallins against heat denaturation. Simple modification of lysine residues by early glycation or carbamylation had no effect. However, late (cross-linking) glycation products and oxidative modifications decreased the chaperone-like activity of alpha-crystallin. Homopolymers of alpha A-crystallin had a higher protecting capacity compared with those of alpha B-crystallin. The in vivo phosphorylated forms of especially alpha A- but also alpha B-crystallin revealed a somewhat better protecting ability than the respective non-phosphorylated forms.

Animals↗
PubMed1995

Rho B-crystallin, an aldose reductase-like lens protein in the gecko Lepidodactylus lugubris.

The ocular lenses of the diurno-nocturnal gecko Lepidodactylus lugubris contain a monomeric 38-kDa protein at a level of 20 to 22% of the total water-soluble protein. Amino acid sequences of peptides from this protein are most similar--up to 72% identity--to mammalian aldose reductase, an NADPH-dependent reductase which normally occurs at house-keeping levels in the eye lens, and which is involved in the development of diabetic cataract. The sequences show 56% identity with rho-crystallin from lenses of the frog genus Rana. It is concluded that different genes from the same superfamily of NADPH-dependent reductases have been recruited to become highly expressed as lens proteins in at least two different evolutionary lineages. To reflect the relationship with frog rho-crystallin, the gecko lens protein is designated as rho B-crystallin. As for frog rho-crystallin, no enzymatic activity could be established for rho B-crystallin in the gecko lens. Up to now, rho B-crystallin has not been detected in lenses of other reptiles or amphibians.

Aldehyde Reductase↗