Casein micelle structure: susceptibility of various casein systems to proteolysis

Proteolytic Activity and Production of γ-Aminobutyric Acid by Streptococcus thermophilus Cultivated in Microfiltered Pasteurized Milk

A set of 191 strains of Streptococcus thermophilus were preliminarily screened for the presence of the genes codifying for cell envelope-associated proteinase (prtS) and for glutamate decarboxylase (gadB) responsible for γ-aminobutyric acid (GABA) production. The growth and proteolytic activity of the gadB-positive strains (9 presenting the prtS gene and 11 lacking it) were studied in microfiltered pasteurized milk. Degradation of both caseins (capillary electrophoresis) and soluble nitrogen fractions (HPLC) and changes in the profile of free amino acids (FAAs; ion-exchange chromatography) were evaluated at inoculation and after 6 and 24 h of incubation at 41 °C. None of the strains was capable of hydrolyzing caseins and β-lactoglobulin, and only two hydrolyzed part of α-lactalbumin, these proteins being present in their native states in pasteurized milk. Contrarily, most strains were able to hydrolyze peptones and peptides. For initial growth, most strains relied on the FAAs present in milk, whereas, after 6 h, prtS⁺ strains released variable amounts of FAA. One prtS⁺ strain expressed a PrtS^- phenotype, and two prtS^- strains showed a rather intense proteolytic activity. Only five strains (all prtS⁺) produced GABA, in variable quantities (up to 100 mg/L) and at different rates, depending on the acidification strength. Addition of glutamate did not induce production of GABA in nonproducing strains that, however, unexpectedly were shown to adopt the degradation of arginine into citrulline and ornithine as an alternative acid resistance system and likely as a source of ATP.

Potential antiviral activities of camel, bovine, and human lactoperoxidases against hepatitis C virus genotype 4

Lactoperoxidases (LPOs) were assayed against hepatitis C virus (HCV) using PCR. Direct interaction of HCV with LPO neutralized the viral particles and prevented entry into cells. LPOs inhibited virus amplification in infected HepG2 cells with a relative activity of 100%.

Mammals, Milk, Molecules, and Micelles

After a brief description of my family background and school days, my professional career as a dairy scientist is described under three headings: research, teaching, and writing. My research activities fall into four areas: biochemistry of cheese, fractionation and characterization of milk proteins, heat stability of milk, and dairy enzymology. Finally, I offer some advice to young scientists.

Proteolysis of αs1-casein by chymosin: influence of pH and urea

The specificity of chymosin on αs1-casein was shown to be dependent on the reaction pH and on the state of aggregation of the substrate. In aqueous solution αs1-casein was optimally hydrolysed to αs1-I at pH 5·8; if the casein was solubilized in the isoelectric region by the use of 5 M-urea, optimum proteolysis occurred at pH 2·8. Hydrolysis of αs1-I to yield αs1-II, αs1-III and αs1-IV occurred at pH values > 5·8 in the presence or absence of urea. In the isoelectric region αs1-II, αs1-III and αs1-IV were not formed in the absence of urea where the substrate was aggregated: instead a peptide αs1-V was produced; at the same pH and using urea as a solubilizing agent αs1-II, αs1-III and αs1-IV were formed together with a further peptide αs1-VI.

Proteolysis of Cabrales cheese and other European blue vein cheese varieties

The extent and level of proteolysis were evaluated in Cabrales and five other European blue vein cheese varieties, Bleu de Bresse, Danablu, Edelpilzkäse, Gorgonzola and Roquefort, by analysing their different N-containing constituents and determining the degree of casein hydrolysis. The electrophoretic patterns of the caseins in the cheeses confirmed their strong proteolysis, components featuring the lowest electrophoretic mobility offering the greatest resistance to subsequent hydrolysis. Component αs1 — I was detected in all varieties studied with the exception of Roquefort cheese.

Probing the location of casein fractions in the casein micelle using enzymes and enzyme–dextran conjugates

The rates of milk clotting and of formation of para-κ-casein in milk, colloidal phosphate-free milk and isolated κ-casein by pepsin and by its soluble size-fractionated conjugates with dextran were determined. Milk clotting with all the enzyme derivatives was dependent on the rate of the enzymic phase and required essentially complete κ-casein hydrolysis at 30 °C and throughout the range of pH 5·6–6·7. κ-Casein hydrolysis by pepsin at pH 6·6 was fastest in milk and slowest in isolated κ-casein, but the rate decreased as the enzyme size increased, especially with milk. When corrected for the changes in pepsin activity, the rates of κ-casein hydrolysis in all substrates were identical at 30 and 5 °C, but increased with decrease in pH, especially with the larger enzyme conjugates. Hydrolysis of the C-terminal bonds of β-and κ-casein in native and disrupted casein micelles by carboxypeptidase A and soluble conjugates of it were also investigated. κ-Casein was hydrolysed much faster, and β-casein slightly faster, in native than in disrupted micelles by the native enzyme. Increase in the size of carboxypeptidase A increased the rate of hydrolysis of κ-casein in disrupted micelles and also induced lag periods before hydrolysis commenced, especially with disrupted micelles. The results are compatible with a model for the casein micelle in which the κcasein is on the outside and the casein components are in a more ordered arrangement than in the casein complexes formed on micelle disruption. They also indicate that immobilized coagulants would be unable to clot milk.

Contribution of rennet and starter proteases to proteolysis in Cheddar cheese

Proteolysis in aseptic, chemically acidified (GDL) cheese and in starter cheese made under controlled bacteriological conditions (i.e. free of non-starter micro-organisms) was measured by gel electrophoresis, the formation of pH 4·6- and 12% TCA-soluble N, gel filtration and the liberation of free amino acids. The results show that rennet was mainly responsible for the level of proteolysis detected by gel electrophoresis, pH 4·6-soluble N and gel filtration i.e. large, medium and small peptides. However, rennet alone was capable of producing only a limited range of free amino acids; only methionine, histidine, glycine, serine and glutamic acid were produced at quantifiable levels (> 0·2 μmoles/g) in GDL cheese; it is suggested that free amino acids in Cheddar cheese are mainly the result of microbial peptidase activity. The levels of free amino acids in the starter cheese were considerably lower than values reported for commercial Cheddar.

Identification of peptides in milk as a result of proteolysis at different levels of somatic cell counts using LC MALDI MS/MS detection

The somatic cell count (SCC) in milk is associated with increasing proteolytic degradation of caseins and it has been suggested that enzymes derived from somatic cells contribute to a lower yield and poorer quality of cheese. It is essential to increase the knowledge on naturally occurring milk proteinase activities to better understand how to improve the technological quality of milk. The aim of this work was to identify peptides actually present in milk as a result of proteolysis at different levels of SCC and to assign these peptides to potential responsible proteases where possible. Peptide fractions were prepared from acid whey by ultrafiltration at a molecular cut-off value of 10 000 Da. The peptides were separated using capillary reversed phase high performance liquid chromatography (RP-HPLC) and identified by matrix-assisted laser desorption/ionization-time of flight tandem mass spectrometry (MALDI-TOF MS/MS). Peptides identified ranged in mass from 1023 to 2000 Da, and originated from αS1-, αS2- or β-casein. Possible responsible proteases that could be suggested when examining the peptide cleavage sites included plasmin, cathepsin B, D and leukocyte elastase. The results indicated that plasmin was primarily responsible for the observed proteolysis in milk at low cell count, whereas the cathepsins and elastase became implicated at elevated cell count. Specificity and activity of cathepsins and elastase has earlier mainly been studied in model systems, whereas less is known about their activities in milk itself. This is also the first indication of involvement of elastase in milk proteolysis through the unequivocal determination of cleavage sites.

Purification and characterization of extracellular caseinolytic enzyme of Micrococcus sp. MCC-315 isolated from cheddar cheese

Micrococcus sp. MCC-315, an organism isolated from Cheddar cheese, produced an extracellular calcium metalloenzyme. This protease was purified to homogeneity from culture supernatant by precipitation with ammonium sulfate (50 to 70% saturation) and gel filtration through Sephadex G-100, resulting in about 82 times increase of specific activity and 53% recovery of the enzyme. The protease exhibited a pH optimum at 10.6 for both whole casein and beta-casein. It had optimum activity for whole casein in the presence and absence of calcium++ at 60 and 50 degrees C, respectively, and at 37 to 40 degrees C for beta-casein with or without calcium++. The enzyme was stable at 45 degrees C but lost activity at higher temperatures. It was inhibited by heavy metal ions but calcium++, cobalt++, manganese++, strontium++, and iron++ had a slight stimulatory effect. The enzyme was inhibited completely and irreversibly by metal chelating agents. Calcium ions were required for maintenance of an active conformation of the enzyme. The enzyme had molecular weight of 28,900 and Michaelis constants 6.66 and 5.00 mg/ml for whole casein and beta-casein. Amino acid analysis of the hydrolyzed enzyme revealed the absence of sulfhydryl groups as was indicated also by lack of inhibition by thiol reagents.

Action of milk-clotting enzymes on beta-caseins from buffalo's and cow's milk

beta-Caseins isolated from buffalo's and cow's milk were hydrolysed either with rennet or with microbial proteases from Mucor miehei, M. pusillus Lindt or Endothia parasitica. The degradation products were separated by polyacrylamide-gel electrophoresis and the residual beta-casein was determined quantitatively after various times. The electrophoretic patterns of the degradation products of buffalo and bovine beta-casein produced by the different enzymes were not identical. beta-Casein of buffalo's milk was hydrolysed by rennet and M. miehei protease at a slower rate than that of cow's milk. The reverse was found with E. parasitica and M. pusillus Lindt proteases. Carbamylation of buffalo beta-casein was found to retard its proteolysis by all the enzymes but particularly by rennet and M. miehei protease.