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PubMed1990

Antigen absorption in rabbit bacterial diarrhea (RDEC-1). In vitro modifications in ileum and Peyer's patches.

Intestinal absorption of antigenic (intact) and degraded beta-lactoglobulin and horseradish peroxidase was studied in rabbits experimentally infected at weaning with the rabbit-specific Escherichia coli strain RDEC-1 (015:NM). Transepithelial fluxes of both proteins were measured at four stages after infection: early, peak, late, and recovery, on segments of ileum and Peyer's patches mounted in Ussing chambers. During this period of observation, absorption of antigenic beta-lactoglobulin and intact horseradish peroxidase decreased significantly in the ileum and Peyer's patches of age-matched controls. No significant age-related decrease was observed in the controls for the transport of these proteins in degraded form. RDEC-1 infection caused, first, a rise in degraded protein fluxes across Peyer's patches only, during the early phase of the disease. Second, during the peak phase of infection, absorption of antigenic beta-lactoglobulin by the ileum rose to 74.0 ng/hr/cm2 (90% confidence interval: 63.1, 86.8) vs 15.8 (5.8, 42.8) in the controls (P = 0.07), and absorption of intact horseradish peroxidase rose to 21.8 (16.5, 26.7) ng/hr/cm2 vs 4.4 (0.7, 26.6) in control ileum, (P = 0.09). These rises, which were similar in Peyer's patches, delayed the physiologic decrease in protein absorption that occurs with age. These results indicate that increased antigen absorption is observed during bacterial diarrhea and that it might interfere with the immune responses of the host.

Animals↗
PubMed1992

Lymphocyte responses to food antigens in food sensitive patients with allergic tension-fatigue syndrome.

Scores of radioallergosorbent test (RAST) for cow's milk or buckwheat flour and proliferative responses of peripheral blood mononuclear cells (PBMCs) to bovine serum albumin and beta-lactoglobulin or buckwheat flour were measured in cow's milk or buckwheat flour sensitive patients with allergic tension-fatigue syndrome. In all 3 cow's milk sensitive patients with allergic tension-fatigue syndrome, RAST scores for cow's milk were negative or slightly positive, but PBMCs well responded to bovine serum albumin and beta-lactoglobulin, but not to ovalbumin. In a buckwheat flour sensitive patient with allergic tension-fatigue syndrome, RAST scores for buckwheat flour were negative, but PBMCs well responded to buckwheat flour, but not to ovalbumin, bovine serum albumin and beta-lactoglobulin. Conversely, in cow's milk or buckwheat flour sensitive patients with immediate allergic symptoms, RAST scores for offending foods were positive although PBMCs did not respond to offending food antigens. These results suggest that proliferative responses of PBMCs to food antigens are very useful for detection of offending foods in allergic tension-fatigue syndrome.

Adolescent↗
PubMed2005

Measurement of the kinetics of protein unfolding in viscous systems and implications for protein stability in freeze-drying.

PURPOSE: The aim of the study is to determine the degree of coupling between protein unfolding rate and system viscosity at low temperatures in systems relevant to freeze-drying. METHODS: The cold denaturation of both phosphoglycerate kinase (PGK) and beta-lactoglobulin were chosen as models for the protein unfolding kinetics study. The system viscosity was enhanced by adding stabilizers (such as sucrose), and denaturant (guanidine hydrochloride or urea) was added to balance the stabilizing effect of sucrose to maintain the cold denaturation temperature roughly constant. The protein unfolding kinetics were studied by both temperature-controlled tryptophan emission fluorescence spectroscopy and isothermal high-sensitivity modulated differential scanning calorimetry (MDSC) (Tzero). Viscometers were used to determine the system viscosity. To verify the predictions of structure based on protein unfolding dynamics, protein formulations were freeze-dried above the glass transition temperatures, and the protein structures in dry products were determined by fluorescence spectroscopy of reconstituted solids by extrapolation of the solution data to the time of reconstitution. RESULTS: Empirical equations describing the effect of sucrose and denaturant (urea and guanidine hydrochloride) on protein cold denaturation were developed based on DSC observations [X. C. Tang and M. J. Pikal. The Effects of Stabilizers and Denaturants on the Cold Denaturation Temperature of Proteins and Implications for Freeze-Drying. Pharm. Res. Submitted (2004)]. It was found that protein cold denaturation temperature can be maintained constant in system of increasing sucrose concentration by simultaneous addition of denaturants (urea and guanidine hydrochloride) using the empirical equations as a guide. System viscosities were found to increase dramatically with increasing sucrose concentration and decreasing temperature. The rate constants of protein unfolding (or the half-life of unfolding) below the cold denaturation temperature were determined by fitting the time dependence of either fluorescence spectroscopy peak position shift or DSC heat capacity increase to a first-order reversible kinetic model. The half-life of unfolding did slow considerably as system viscosity increased. The half-life of PGK unfolding, which was only 3.5 min in dilute buffer solution at -10 degrees C, was found to be about 200 min in 37% sucrose at the same temperature. Kinetics of protein unfolding are identical as measured by tryptophan fluorescence emission spectroscopy and by high-sensitivity modulated DSC. The coupling between protein unfolding kinetics and system viscosity for both proteins was significant with a stronger coupling with PGK than with beta-lactoglobulin. The half-lives of PGK and beta-lactoglobulin unfolding are estimated to be 5.5 x 10(11) and 2.2 years, respectively, even when they are freeze-dried in sucrose formulations 20 degrees C above Tg'. Thus, freeze-drying below Tg' should not be necessary to preserve the native conformation. In support of this conclusion, native PGK was obtained after the freeze-drying of PGK at a temperature more than 60 degrees C above the system Tg' in a thermodynamically unstable system during freeze-drying. CONCLUSIONS: Protein unfolding kinetics is highly coupled with system viscosity in high viscosity systems, and the coupling coefficients are protein dependent. Protein unfolding is very slow on the time scale of freeze-drying, even when the system is freeze-dried well above Tg'. Thus, it is not always necessary to freeze-dry protein formulations at temperature below Tg' to avoid protein unfolding. That is, protein formulations could be freeze-dried at product temperature far above the Tg', thereby allowing much shorter freeze-drying cycle times, with dry cake structure being maintained by the simultaneous use of a bulking agent and a disaccharide stabilizer.

Calorimetry, Differential Scanning↗
PubMed1989

Influence of protein conformation on its adaptability under chaotropic conditions.

The thermostability of serum albumin and beta-lactoglobulin in various salt solutions was studied using differential scanning calorimetry. Below 1.0 M salt concentrations, the relative effectiveness of various sodium salts on increasing the thermostability of beta-lactoglobulin followed the classic Hofmeister or lyotropic series, i.e. SO2-(4) greater than Cl- greater than Br- greater than ClO-4 greater than SCN-; however, in the case of serum albumin the above order was reversed, i.e. ClO-4 greater than SCN- greater than Br- greater than Cl- greater than SO2-(4), indicating that the thermostability of serum albumin was higher in chaotropic solution conditions. Circular dichroic analysis of serum albumin in NaClO4 solutions revealed that the alpha-helical content of the protein increased from 59% to 73% in 1.0 M NaClO4; no similar increase in secondary structure was observed for beta-lactoglobulin. These observations contradicted the general notion that the chaotropic effect of neutral salts on the stability of macromolecules is independent of any details of the macromolecular conformation itself. The results presented here indicate that the predisposition of the native conformation of a protein per se might affect whether the protein would undergo stabilization or destabilization (i.e. conformational adaptability) under moderate chaotropic solution conditions.

Animals↗
PubMed1990

Interaction of bovine lactoferrin with other proteins of milk whey.

The association between bovine lactoferrin and the major bovine whey proteins, beta-lactoglobulin, alpha-lactalbumin and albumin has been studied by immunochemical techniques, gel filtration and affinity chromatography in lacteal secretions and using purified proteins. Bovine lactoferrin is able to form non-covalent complexes with beta-lactoglobulin or albumin, with lactoferrin-protein molar ratios of 2:1 and 1:1 respectively. No association was detected with alpha-lactalbumin. Lactoferrin interacts with beta-lactoglobulin-Sepharose at low ionic strength, but not in the presence of 0.3 M NaCl, indicating that ionic interactions are important. The lack of association with alpha-lactalbumin suggests however a certain degree of specificity in this electrostatic interaction.

Albumins↗
PubMed1994

Differential effects of milk proteins, BSA and soy protein on 4NQO- or MNNG-induced SCEs in V79 cells.

The possible antimutagenic effects of five different proteins against the mutagen 4-nitroquinoline 1-oxide (4NQO) were assessed in a mammalian cell system, using the sister chromatid exchange (SCE) test in Chinese hamster cells (V79). For this purpose the proteins casein, bovine serum albumin (BSA), soy protein, total whey protein and beta-lactoglobulin were studied, as well as pepsin-hydrolysed casein. In addition, the effect of casein on 1-methyl-1-nitroso-3-nitroguanidine (MNNG) was studied. The proteins were tested at a concentration of 1.15% (w/v). Casein was studied over a concentration range of 0 to 1.15% (w/v). A non-toxic concentration was used for the mutagens. Casein hydrolysis by pepsin took place in vitro, simulating human stomach conditions, which resulted in 84% non-casein-N and 16% remaining casein-N. Casein significantly inhibited SCE induction by 4NQO (inhibition 78%, at 1.15% casein, P < 0.05) and by MNNG (83%, at 1.15% casein, P < 0.01). BSA also significantly inhibited 4NQO-induced SCEs (94%, at 1.15% BSA; P < 0.01). However, soy protein, the whey protein fraction of milk and beta-lactoglobulin showed no inhibitory effects. Pepsin-hydrolysed casein inhibited SCE induction by 4NQO and MNNG to a similar extent as non-hydrolysed casein. It is concluded that casein, its pepsin hydrolysis products and BSA may protect mammalian cells against certain genotoxic compounds, whereas other milk proteins, such as whey protein, beta-lactoglobulin and soy protein, do not have this protective action. Although the mechanism of antimutagenicity is unknown, it seems plausible that the protein acts as a blocking agent by chemical or physical interaction with the mutagens. The accessibility of protein molecules and the presence of nucleophilic binding sites may be decisive factors in determining antimutagenic properties of proteins.

4-Nitroquinoline-1-oxide↗
PubMed2001

Effect of buffer concentration on gradient chromatofocusing performance separating protiens on a high-performance DEAE column.

Gradient chromatofocusing is a recently developed chromatographic technique that overcomes the limitations of conventional chromatofocusing. This technique employs a HPLC gradient system and simple low-molecular-mass buffer components to generate linear or other function pH gradients on ion-exchange columns. Results of the present work show a superior separation of beta-lactoglobulin A and B in gradient chromatofocusing compared to salt gradient chromatography using the same DEAE column, with an optimized resolution of 2.3 obtained with gradient chromatofocusing compared to 1.1 obtained with NaCl gradients at constant pH. A significant advantage of the gradient chromatofocusing technique over the conventional chromatofocusing technique is its ability to employ a relatively wide range of buffer concentrations in the mobile phase, the effect of which is studied in the present work. Five proteins (conalbumin, ovalbumin, bovine serum albumin, beta-lactoglobulin A and B) are chromatographed on a DEAE-polymethacrylate HPLC anion-exchange column using the same approximately linear pH gradient profile but different mobile phase buffer concentrations. Results show a significant effect of buffer concentration on peak width, separation factor and resolution. For example, resolution increases from 1.5 to 2.3 in the separation of beta-lactoglobulin A and B when the concentration of each of the components in the 100% elution buffer is increased from 6.25 to 25.0 mM (with the same outlet pH gradient). This separation trend is also seen in the chromatography of ovalbumin from a commercial source, noting a progressive increase in resolution of two peaks in the sample (resolution increased from 0.7 to 2.4) when the concentration of each of the components in the 100% elution buffer is increased from 6.25 to 37.5 mM (same outlet pH gradient). The gains in the resolution are attributed to an increase in the separation factor, since the peak widths are generally noted to also increase with increased buffer concentration. These results point to a significant interplay between buffer concentration and pH, which is not effectively exploited in either conventional chromatofocusing or in conventional ion-exchange chromatographic procedures employing salt gradient elution at constant pH. Gradient chromatofocusing has the ability of optimizing both parameters, thus providing it with unique capabilities in protein separations.

Buffers↗
PubMed1997

Comparison of anion-exchange and hydroxyapatite displacement chromatography for the isolation of whey proteins.

In displacement chromatography, several substances may be isolated and concomitantly concentrated, which makes this separation procedure attractive for the processing of diluted product streams containing a number of high value substances. Here, the suitability of anion-exchange and hydroxyapatite displacement chromatography for the processing of technical dairy whey is investigated. The pH and flow-rate of the carrier, the displacer chemistry and, in case of the apatite, the particle diameter of the stationary phase are considered. As a consequence of the pH sensitivity of the beta-lactoglobulin, one major whey component, apatite displacement chromatography is less then successful in whey separation. At a denaturing carrier pH (> 8.5) the beta-lactoglobulin zone is broad and stretches over the entire displacement train. At a lower carrier pH, previously successful polyanionic displacers do not bring about separation, while low-molecular-mass ones do, but they tend to overrun and thus contaminate the protein zones. In the case of anion-exchange displacement chromatography, polyanions, especially polyacrylic acid (PAA, M(r) 6000), constitute suitable displacers. Here too, a carrier pH of 8.0 is most suited to the separation of the whey proteins. The low-molecular mass-displacer iminodiacetic acid (IDA, M(r) 133.4), on the other hand, displaces only alpha-lactalbumin. The beta-lactoglobulin remains on the column. PAA is used as the displacer to process a dairy whey sample.

Animals↗
PubMed2003

Disproportionation of clustered protein-stabilized bubbles at planar air-water interfaces.

The rate of shrinkage of air bubbles, of initial radius from 50 to 200 microm, injected beneath a planar air-water interface has been measured. Bubbles were stabilized in solutions of 0.05 wt% gelatin or pure beta-lactoglobulin. It has been observed that small size differences between two closely spaced or touching bubbles result in markedly divergent rates of shrinkage for the two. By studying a number of different initial bubble configurations, it is demonstrated that the overall change in bubble size distribution is strongly dependent on local, interbubble gas diffusion. In this respect, the strong tendency for the gelatin-stabilized bubbles to aggregate and shrink, while remaining in contact, produced patterns of disproportionation significantly different from those observed with beta-lactoglobulin. In beta-lactoglobulin solutions, it was usually found that bubbles initially in contact shrank away from each other with time, becoming increasingly isolated as a result. A theoretical approach that can exactly incorporate the perturbation of local diffusion fluxes due to the proximity of two bubbles is presented. This enables one to map a "stability diagram" that delineates regions where the larger bubble of a pair will either shrink or grow, according to the relative size of the bubbles and their separation. Theoretical calculations show that it is possible for a bubble to exhibit more complex shrinkage behavior than is predicted by a mean field approach or the Lifshitz, Slyozov, and Wagner (LSW) theory of Ostwald ripening for dilute systems. The inclusion of dilatational elasticity in the theoretical model introduces additional complications, which are also briefly discussed.

Air↗
PubMed1992

Immunologic changes associated with the development of tolerance in children with cow milk allergy.

The purpose of this study was to determine whether cow milk-specific antibody responses correlated with the development of clinical tolerance in cow milk-allergic children. Double-blind, placebo-controlled food challenges were performed annually in 29 patients with cow milk allergy. Clinical reactivity was lost in 11 (38%) of 29 patients. The median age for all patients at the time of diagnosis by these food challenges was 3 years; more than 80% of patients in each group had atopic dermatitis as part of their presenting symptoms. Casein-specific and beta-lactoglobulin-specific IgE, IgG, IgG1, and IgG4 antibody concentrations were analyzed in all patients at regular intervals. In the patients becoming clinically tolerant to cow milk, the IgE-specific antibody concentrations and IgE/IgG-specific ratios for both milk proteins were lower initially and decreased significantly with time, in comparison with those in the group who retained clinical sensitivity. The concentrations of IgG1- and IgG4-specific antibody to casein and the IgE/IgG1 and IgE/IgG4 ratios for both casein and beta-lactoglobulin were significantly less in the patients losing clinical reactivity. No differences in the IgG-specific concentrations were observed in either group at any of the evaluation times noted above. Monitoring similar casein-specific and beta-lactoglobulin-specific IgE concentrations and IgE/IgG ratios may help predict which patients will ultimately lose their clinical reactivity to cow milk.

Adolescent↗
PubMed1986

Cow milk protein antigens and antibodies in serum of premature infants during the first 10 days of life.

Antigenic beta-lactoglobulin and alpha-casein were measured in the sera of 45 formula-fed infants of 31 to 41 weeks of gestation at 5 days and at 10 days of age. Quantitation was performed by a sensitive ELISA inhibition assay. On day 5 of life antigenic lactoglobulin was detected in 14 of 19 infants of less than 37 weeks gestation, but in only one of 10 infants of more than 36 weeks gestation. On day 10 of life the sera of all infants contained antigenic lactoglobulin. In contrast, on day 5 antigenic casein was present in four of 17 infants of less than 37 weeks gestation, but in 10 of 12 infants of the more mature group. On day 10 casein was detected in seven of 28 infants, with no difference between groups; anti-casein was found in eight of 12 infants. Infants of less than 37 weeks gestation have different absorption patterns than more mature infants do. "Gut closure" is an unlikely explanation for these findings.

Animals↗
PubMed2000

First demonstration of an inhibitory activity of milk proteins against human immunodeficiency virus-1 reverse transcriptase and the effect of succinylation.

A variety of milk proteins including lactoferrin, angiogenin-1, alpha-lactalbumin, beta-lactoglobulin, lactoperoxidase, casein and the novel whey proteins lactogenin and glycolactin were tested for inhibitory activity toward human immunodeficiency virus-1 reverse transcriptase (HIV-1 RT), alpha-glucosidase, beta-glucosidase and beta-glucuronidase. Lactoferrin exerted the most potent inhibitory action with an IC50 of about 6 microM. Lactoperoxidase, lactogenin, angiogenin-1 and glycolactin inhibited HIV-1 RT activity with decreasing potencies. Beta-lactoglobulin, alpha-lactalbumin and casein displayed little or no inhibitory effect. Succinylation with succinic anhydride augmented the inhibitory effect of glycolactin, beta-lactoglobulin, alpha-lactalbumin, casein and human lactoferrin. The inhibitory effect of the various milk proteins on the activities of alpha-glucosidase, beta-glucosidase and beta-glucuronidase was meager. Succinylation tended to increase the alpha-glucosidase-inhibitory effect of milk proteins but neither their beta-glucosidase-inhibitory nor beta-glucuronidase-inhibitory effect was affected.

Animals↗
PubMed1997

Dot blot chemiluminescence assay for studying food protein binding to small intestinal brush border membranes in vitro.

Interactions of food proteins with the apical membrane of small intestinal epithelial cells can influence enterocytic antigen handling. For studying these interactions in vitro, isolated brush border membrane vesicles are a widely accepted model. In order to improve measurement of food protein binding, we developed a sensitive dot blot chemiluminescence assay. This assay comprises immobilization of membrane vesicles on nitrocellulose, detection of bound biotinylated food proteins by a peroxidase-catalyzed chemiluminescence reaction, and densitometric quantitation of signal intensities. By using this assay, saturation of brush border membrane binding of food proteins (gliadin peptides, alpha-casein, beta-lactoglobulin, ovalbumin) was demonstrated. Inhibition studies indicated components of specific membrane binding of gliadin peptides, alpha-casein and beta-lactoglobulin, whereas aggregation tendency of ovalbumin interfered with inhibition experiments. Maximal binding intensities of gliadin peptides (22.2 +/- 1.2 densitometric units (d.u.)/microgram membrane protein), alpha-casein (27.9 +/- 1.7 d.u./microgram) and ovalbumin (21.3 +/- 1.6 d.u./microgram) were comparable to sugar-specific lectin binding (range from 23.4 to 35.1 d.u./microgram), in contrast to significantly less binding of beta-lactoglobulin (6.8 +/- 0.6 d.u./microgram). The dot blot chemiluminescence assay is appropriate for characterizing interactions between food proteins and brush border membranes. Its sensitivity makes investigation of pathological membrane alterations possible. Besides, it might be useful for any studies defining ligand-membrane interactions.

Animals↗
PubMed1975

The precipitation of proteins by carboxymethyl cellulose.

Carboxymethyl cellulose (CMC) formed insoluble complexes with beta-lactoglobulin, bovine serum albumin and Na caseinate. Maximum precipitation of the beta-lactoglobulin-CMC complex occurred at pH 3-2, whereas maximum precipitation of the bovine serum albumin. The percentage of protein precipitated by CMC decreased with increasing ionic strength of the solution, the rate of decrease being least for bovine serum albumin. At a given ionic strength, more protein was precipitated by CMC of high degree of substitution than by CMC of low degree of substitution. The change in pH (delta pH) occurring on mixing CMC and unbuffered protein solutions, each initially at the same pH, was measured. delta pH was negative for beta-lactoglobulin-CMC mixtures over the pH range 7--2 (minimum at pH 5-5). For bovine serum albumin--Cmc and Na caseinate--CMC mixtures, delta pH was positive between pH 7 and 3-2 (maximum at pH 4-5), zero at pH 3-2 and negative between pH 3-2 and 2-0 (minimum at pH 2-8).

Animals↗
PubMed2000

High-pressure treatment of milk: effects on casein micelle structure and on enzymic coagulation.

High isostatic pressures up to 600 MPa were applied to samples of skim milk before addition of rennet and preparation of cheese curds. Electron microscopy revealed the structure of rennet gels produced from pressure-treated milks. These contained dense networks of fine strands, which were continuous over much bigger distances than in gels produced from untreated milk, where the strands were coarser with large interstitial spaces. Alterations in gel network structure gave rise to differences in rheology with much higher values for the storage moduli in the pressure-treated milk gels. The rate of gel formation and the water retention within the gel matrix were also affected by the processing of the milk. Casein micelles were disrupted by pressure and disruption appeared to be complete at treatments of 400 MPa and above. Whey proteins, particularly beta-lactoglobulin, were progressively denatured as increasing pressure was applied, and the denatured beta-lactoglobulin was incorporated into the rennet gels. Pressure-treated micelles were coagulated rapidly by rennet, but the presence of denatured beta-lactoglobulin interfered with the secondary aggregation phase and reduced the overall rate of coagulation. Syneresis from the curds was significantly reduced following treatment of the milk at 600 MPa, probably owing to the effects of a finer gel network and increased inclusion of whey protein. Levels of syneresis were more similar to control samples when the milk was treated at 400 MPa or less.

Animals↗
PubMed1976

A kinetic study of protein-protein interactions.

Kinetic studies have been carried out of the monomer-dimer interaction of insulin, beta-lactoglobulin, and alpha-chymotrypsin using stopped-flow and temperature-jump techniques. The pH indicators bromothymol blue, bromophenol blue, and phenol red were used to monitor pH changes associated with the monomer-dimer interaction. In all three cases a kinetic process was observed which could be attributed to a simple monomer-dimer equilibrium, and association (k1) and dissociation (k-1) rate constants were determined. The results obtained are as follows: for insulin at 23 degrees C, pH 6.8, 0.125 M KNO3, k1 = 1.14 X 10(8) M-1 s-1, k-1 - 1.48 X 10(4)s(-1); for beta-lactoglobulin AB at 35 degrees C, pH 3.7, 0.025 M KNO3, d1 = 4.7 X 10(4) M-1 s-1, k-1 = 2.1 s-1; for alpha-chymotrypsin at 25 degreesC, pH 4.3, 0.05 M KNO3 k1 - 3.7 X 10(3) M-1 s-1, k-1 - 0.68 s-1. The kinetic behavior of the separated beta-lactoglobulin A and B was similar to that of the mixture. In the case of chymotrypsin, bromophenol blue was found to activate the enzyme catalyzed hydrolysis of p-nitrophenyl acetate, and a rate process was observed with the temperature jump which could be attributed to a conformational change of the indicator-protein complex. The association rate constant for dimer formation of insulin approaches the value expected for a diffusion-controlled process, while the values obtained for the other two proteins are below those expected for a diffusion-controlled reaction unless unusally large steric and electrostatic effects are present.

Binding Sites↗
PubMed2001

In situ measurement of the displacement of protein films from the air/water interface by surfactant.

The displacement of spread protein films from the air/water interface by surfactant was followed using Brewster angle microscopy (BAM) and interfacial rheology. The displacement of beta-lactoglobulin and beta-casein by a nonionic surfactant was monitored as a function of both surface pressure and time. In both cases, protein displacement occurred over the same surface pressure range that had been observed previously by atomic force microscopy (AFM). In the case of the beta-lactoglobulin, surfactant domains grew large enough in the protein film to be visible in the BAM images. The shapes of the domains were very similar to those seen previously by AFM in the late stages of displacement. The results from both proteins confirm the results published previously while highlighting some implications for the application of the "orogenic" model of displacement for large protein films. The surface rheological data showed that the beta-lactoglobulin/surfactant mixed film retained much of its elasticity until the latter stages of displacement. This indicates that at least in the early stages of displacement, the mixed film was dominated by the behavior of the protein in the film.

Air↗
PubMed2000

Lactoperoxidase-induced protein oxidation in milk.

The reaction between lactoperoxidase (LPO) and H(2)O(2) in the presence of bovine serum albumin (BSA), beta-lactoglobulin, or casein was investigated for the formation of protein radicals by freeze-quench electron spin resonance (ESR) and by the formation of the protein oxidation product, dityrosine. The presence of BSA resulted in a dramatic change after 1 min of reaction in the obtained ESR spectrum compared with the spectrum obtained for LPO and H(2)O(2) alone. Furthermore, experiments employing BSA or beta-lactoglobulin resulted in the formation of long-lived protein radicals detectable 10 min after initiation of the reaction. The presence of casein resulted in a minor change in the fine structure of the ESR spectrum after 1 min of reaction compared with LPO and H(2)O(2) alone, but no difference between the two reaction mixtures could be observed after 10 min of reaction. The formation of dityrosine could be detected in reaction mixtures containing LPO and H(2)O(2) after 1 and 10 min of incubation at 25 degrees C both in the absence and in the presence of BSA, beta-lactoglobulin, or casein. The presence of casein resulted in an increased dityrosine concentration compared with the reaction with LPO and H(2)O(2) alone. Endogenous LPO in unpasteurized milk was activated at 25 degrees C by adding 1 mM H(2)O(2). Radical species could be detected directly in the milk by freeze-quench ESR during the initial phase of the reaction, and dityrosine could be measured after 4 h of incubation. The role of LPO activity in the formation of ESR detectable radical species and dityrosine in milk was further verified in ultrahigh temperature (UHT) milk with no endogenous enzyme activity, as the formation of ESR detectable radical species and dityrosine took place in UHT milk only upon the addition of both H(2)O(2) and exogenous LPO.

Animals↗