Isolation of kappa-casein-like proteins from milks of various species

Can Recombinant Tree Shrew (Tupaia belangeri chinensis) Chymosin Coagulate Cow (Bos taurus) Milk?

Abstract

Genetically engineered chymosin from the tree shrew (Tupaia belangeri chinensis) has been obtained and partially characterized for the first time. The target enzyme was produced in Escherichia coli, strain BL21(DE3). It was shown that tree shrew recombinant chymosin coagulates cow milk (Bos taurus). The total and specific milk-clotting activity of the obtained enzyme was 0.7–5.3 IMCU/mL and 8.8–16.6 IMCU/mg. The nonspecific proteolytic activity of tree shrew recombinant chymosin in relation to total bovine casein was 30 and 117% higher than that of recombinant chymosin of cow and of single-humped camel respectively. It was found that in comparison with most of the known genetically engineered chymosins, the tree shrew enzyme showed exceptionally low thermal stability. After heating at 45°C, the coagulation ability of tree shrew recombinant chymosin decreased by more than 40%, and at 50°C the enzyme lost more than 90% of the initial milk-clotting activity. The Michaelis constant (Km), enzyme turnover number (kcat), and catalytic efficiency (kcat/Km) for genetically engineered chymosin from the tree shrew were 6.3 ± 0.1 µM, 11 927 ± 3169 s–1 and 1968 ± 620 µM–1 s–1, respectively. Comparative analysis showed that the primary structure of the chymosin-sensitive site of cow kappa-casein and the supposed similar sequence of tree shrew kappa-casein differed by 75%. The ability of tree shrew recombinant chymosin to coagulate cow’s milk, along with a low thermal stability and high catalytic efficiency with respect to the substrate, imitating the chymosin-sensitive site of cow kappa-casein, suggests that this enzyme is of potential interest for cheese making.

Proteomic comparison of equine and bovine milks on renneting

Rennet-induced coagulation of bovine milk is a complex mechanism in which chymosin specifically hydrolyzes κ-casein, the protein responsible for the stability of the casein micelle. In equine milk, this mechanism is still unclear, and the protein targets of chymosin are unknown. To reveal the proteins involved, the rennetability of equine milk by calf chymosin was examined using gel-free and gel-based proteomic analysis and compared to bovine milk. RP-HPLC analysis of bovine and equine milks showed the release of several peptides following chymosin incubation. The hydrolyses of equine and bovine casein by chymosin were different, and the major peptides produced from equine milk were identified by mass spectrometry as fragments of β-casein. Using two-dimensional electrophoresis, equine β-casein was confirmed as the main target of calf chymosin over 24 h at 30 °C and pH 6.5. The gel-based analysis of equine milk discriminated between the different individual proteins and provided information on the range of isoforms of each protein as a result of post-translational modifications, as well as positively identified for the first time several isoforms of κ-casein. In comparison to bovine milk, κ-casein isoforms in equine milk were not involved in chymosin-induced coagulation. The intensity of equine β-casein spots decreased following chymosin addition, but at a slower rate than bovine κ-casein.

Separation and characterization of mares' milk alpha(s1)-, beta-, kappa-caseins, gamma-casein-like, and proteose peptone component 5-like peptides

The equine alpha(s1)- and beta-caseins (CN) were purified by chromatography on DEAE-cellulose and by reversed-phase HPLC. The alpha(s1)-, beta-, and kappa-CN were characterized either by monodimensional urea-PAGE or sodium dodecylsulfate (SDS)-PAGE or by bidimensional electrophoresis. Kappa-casein was characterized after electrophoresis by glycoprotein-specific staining. To identify alpha(s1)-CN without ambiguity, internal sequences were determined after trypsin or chymosin digestion of purified alpha(s1)-CN. These sequences, that could be estimated to correspond to 62% of the full protein, presented strong identities with regions of alpha(s1)-CN primary structures of other species. In particular, 51, 48, 43, and 40% identities were obtained with corresponding regions of sow, dromedary, cow, and human alpha(s1)-CN, respectively. On the other hand, trace amounts of equine gamma-CN-like and proteose peptone component 5-like peptides were found in the whole CN. They were identified by microsequencing and corresponded to beta-CN peptides generated by plasmin action on the whole CN. The equine alpha(s1), beta-, and kappa-CN were separated by bidimensional electrophoresis in numerous isoelectric variants with apparent isoelectric points distributed between pH 4.4 to 6.3, 4.4 to 5.9, and 3.5 to 5.5, respectively. The beta- and kappa-CN displayed a more acidic character in the mare than in the cow.

Isolation and characterization of beta- and gamma-caseins from horse milk

Three groups of casein components were isolated from horse milk. Group I is almost insoluble at acid and neutral pH, and is rather heterogeneous on alkaline gels with or without sodium dodecyl sulphate. Group II shows strong similarity to beta-casein from other species, as concluded from its amino acid composition and its N- and C-terminal sequences. This group consists of five electrophoretically distinguishable forms, all containing ester phosphate groups but no carbohydrate. Group III is composed of C-terminal fragments of the beta-like (group II) fraction and probably arises from the action of a plasmin-like enzyme present in horse milk. It does not contain phosphate or carbohydrate. Homology of this group with bovine gamma-caseins is demonstrated. Both beta- and gamma-like caseins are more soluble at 4 degrees C than at room temperature.

Purification and characterization of four components of rat caseins

Rat casein components (C1-, C3A-, C3B- and C4-casein) were extensively purified from rat milk, and the properties of these proteins were compared with those of other caseins including rat C2-casein. C1-casein was precipitated by a low concentration of CaCl2 (1.5 mM). Both C3A- and C3B-casein were less sensitive to Ca2+ than were C1- and C2-casein, and the presence of 20 mM CaCl2 was required at 37 degrees C for their precipitation. C4-casein was absolutely insensitive to Ca2+. This protein exhibited the ability to stabilize all of the other rat casein components against Ca2+-dependent precipitation. In addition, C4-casein contained sialic acid, galactose and N-acetylgalactosamine. Therefore, C4-casein appears to be a bovine kappa-casein-like protein.

Comparative aspects of milk proteins

The current status of knowledge of the major proteins of milks of various species is evaluated. Most of the non-bovine milk proteins are homologous with the recognized families of those of Bos taurus, alpha S1-caseins, alpha S2-caseins, beta-caseins, kappa-caseins, beta-lactoglobulins, and alpha-lactalbumins, each family representing a separate genetic locus specific to the mammary gland. No prominent milk protein not homologous to one of these families has yet been discovered in milk of any species. Genetic polymorphism resulting from substitutions in the polypeptide chains and various degrees of post-translational phosphorylation, glycosylation, and proteolysis have been identified in milk proteins of several species. Total protein production ranges among species from about 0.5 to 10 g/d per kg0.75 maternal weight. Proportions of the several proteins vary greatly among species, but few accurate analytical data are available except for total casein and total whey protein contents.