Comparative micelle structure

Some aspects of the ethanol stability of caprine milk

Caprine skim-milk exhibits markedly lower ethanol (EtOH) stability than bovine skim-milk but can still be characterized by a sigmoidal pH profile. As with bovine milk, the position of this profile along the pH-axis was found to be sensitive to available Ca levels. Manipulation of salt levels, either by serum interchange, addition or diminution did not result in any significant increase in the EtOH stability high pH asymptote, Smax, and reduction of the colloidal calcium phosphate of caprine milk also had no significant effect. EtOH stability/pH profiles similar to those of bovine milk were achieved only by chemical modification of the caprine milk protein by reaction with aldehydes and anhydrides. It is concluded that the low EtOH stability of caprine milk as compared with bovine milk is due to the different proportions of the individual caseins present, in particular the lack of an αsl-casein homologue in caprine milk.

Proteolytic specificity of chymosin on bovine αs1-casein

In dilute buffers ⋟ pH 5·8, chymosin hydrolysed bovine αs1-casein to αs1-I, αs1-II and αs1-III/αs1-IV in a sequential manner while at pH 4·6 αs1-casein was hydrolysed to αs1-I which was then hydrolysed to αs1-V. In the presence of 5 % (w/v) NaCl at pH 5·2, αs1-casein was hydrolysed to αs1-I which was then hydrolysed to αs1-VII and αs1-VIII. αs1-I, αs1-II and αs1-III/αs1-IV were isolated by chromatography on cellulose phosphate followed by preparative slab-gel electrophoresis; αs1-V was isolated by repeated preparative slab-gel electrophoresis and αs1-VII by gel filtration on Sephadex G-150 followed by preparative slab-gel electrophoresis. The mol. wts of the peptides, estimated by gel filtration on Sephadex G-100, were 21000, 17600, 15600, 19900 and 14600 for αs1-I, αs1-II, αs1-III/αs1-IV and αs1-V and αs1-VII respectively. Characterization of the peptides by amino acid, phosphorus and terminal residue analysis showed that they probably consisted of segments of the αs1-casein chain as follows: αs1-I: residues 24/25–199; αs1-II: residues 24/25–169; αs1-III/αs1-IV: residues 24/25–149–150; αs1-V: residues 29/33–199; αs1-VII: residues 56–179. Peptide bonds close to phosphate residues on the αs1-casein chain were not hydrolysed by chymosin at high pH values (⋟ 5·8) when the phosphate groups were charged, but became available for hydrolysis when the reaction pH was reduced. Proteolytic specificity was also modified by NaCl.

Subunit components of casein micelles from bovine, ovine, caprine and equine milks

Subunit components of ovine, caprine and equine casein micelles were separated by gel chromatography using a TSK-G4000SW high-performance column and the subunit components of the fractions analysed and compared with bovine casein. Molecular weights of the casein complexes were determined by the combined use of high-performance gel chromatography and low-angle laser light scattering. The caprine and ovine caseins were separated into three peaks (F2, F3 and F4) which were similar to those of bovine casein with respect to composition and molecular weight (500, 100 and 23 K). These F2, F3 and F4 peaks consisted mainly of αs- and κ-casein, αs- and β-casein and β-casein respectively. The equine casein was separated into two components corresponding to F3 and F4 of bovine casein. These F3 and F4 peaks consisted mainly of αs- and β-casein and β-casein respectively. The molecular weight of equine F3 (850 K) was different from that of the other three species. The contents of F2 and F4 in these caseins were dependent on the contents of κ-casein and β-casein respectively.

Isolation and characterization of monoclonal antibody directed against bovine alpha s2-casein

A monoclonal antibody 62-1A of isotype IgM, directed against bovine alpha s2-casein, was isolated and characterized. Monoclonal antibody 62-1A recognized bovine alpha s2-11P- and alpha s2-9P-caseins by indirect solid phase radioimmunoassay and Western blot analysis. Crossreactivity toward native genetic variants (A, B, and C) of alpha s1-casein was similar but lower (approximately 60 to 70%) than that for alpha s2-11P-casein). Little or no crossreactivity was observed for other bovine milk or serum proteins. Antibody affinity for alpha s2-11P-casein was 1.3 x 10(9)/M. As little as 25 ng/ml (.5 ng/well) of alpha s2-11P-casein was detected by solid phase radioimmunoassay.

Isolation and characterization of new proteins produced by the infusion of colchicine in goat mammary gland

Three new proteins have now been isolated from goat milk obtained after colchicine is infused into the mammary gland. Two of the proteins are proline-rich, and a third is a very acidic phosphoprotein. One of the proline-rich proteins is related compositionally to a sheep colostrum proline-rich protein, which has been shown to have a regulatory effect on the immune response (Janusz, M., Stavoscik, K., Zimecki, M., Wieczorek, Z., and Lisowski, J. (1981) Biochem. J. 199, 9-15). Other aspects of colchicine-treated milks are described.

Purification and properties of a major casein component of rat milk

A casein component (C2-casein) was purified by ion-exchange and gel filtration chromatography from rat milk, and the properties of this protein were examined. The molecular weight of C2-casein, as determined by Sepharose 4B gel filtration in 6 M guanidine hydrochloride, was 34 000 +/- 1000. The average hydrophobicity calculated from the amino acid composition showed that C2-casein is a rather hydrophilic protein. The alpha-helix content obtained from optical rotatory dispersion experiments was about 12%. In ultracentrifugation analyses, monomer and polymer peaks of C2-casein were both seen, and the monomer-to-polymer ratio was not affected by changing temperature conditions. C2-casein was precipitated by the presence of 2.5 mM CaCl2, and the precipitability was greatly decreased by the dephosphorylation of the protein. C2-casein was stabilized from Ca2+-dependent precipitation by the addition of another rat casein component (C3-casein) or of bovine kappa-casein.

Isolation and characterization of porcine beta-casein

Porcine beta-casein was isolated by chromatography on DEAE-cellulose. The protein had a molecular weight of 24 900 as determined by gel filtration on Sephadex G-100 in guanidine-HCl. Its amino acid composition differed from bovine beta-casein especia-ly in respect to serine, alanine and leucine. In common with bovine beta-casein the N-terminal amino acid was arginine; the C-terminal was either alanine or valine, while the C-terminal of bovine beta-casein is valine. At any temperature porcine beta-casein was more sensitive to Ca2+ than bovine beta-casein, while at a fixed Ca2+ concentration porcine beta-casein aggregated at a lower temperature than bovine beta-casein. Porcine beta-casein was susceptible to hydrolysis by calf chymosin but the proteolytic specificity differed from that of calf chymosin on bovine beta-casein.

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.