Studies on β-Lactoglobulins A, B, and C. I. Comparison of Chemical Properties^*

Abstract

@-Lactoglobulins A, B, and C have been isolated, purified, and examined for amino acid composition, The previously described difference between @-A and P-B in content of aspartic acid, glycine, valine, and alanine is confirmed, and @-C is found to differ from P-B only in content of glutamine and histidine. P-Lactoglobulin C has 2 residues more of histidine and 2 residues fewer of glutamine than P-B per molecule of -36,000 molecular weight. DEAE-cellulose column chromatography, using a phosphate buffer containing a NaCl gradient, has been extended to provide a separa-

Studies on genetic variants of α‐lactalbumin and β‐lactoglobulin from milk of native Portuguese ovine and caprine breeds

“…For all types of β‐Lg, except bovine β‐Lg B ( p = 0.5639), significant differences existed between samples with or without NaCl, thus showing that the presence of salt decreases the denaturation onset temperature significantly; this is so probably because salts promote changes of the folded structure of whey proteins via new balances between salting‐in of polar groups and salting‐out of non‐polar groups ( de Wit, 1988). By the same token, the differences observed between β‐Lg B and β‐Lg A could be attributed to different charges brought about by the carboxyl groups of the two additional Asp residues in β‐Lg A ( Kalan et al ., 1965 ), which lead to an important reduction in the denaturation onset temperature of β‐Lg A in the presence of salt. Observation that the unfolding transition of β‐Lg A occurs at lower temperatures than that for β‐Lg B was also reported by Huang et al .…”

Section C. Chemistry of milk proteins

“…Including this assignment, the positions of 14 amides are known, thus completing their assignment (374), or leaving one more to be assigned (46). There is also some doubt about residue 116; it is glutamic acid according to G. Frank (personal communication) or glutamine according to 2 other groups (36,46,237,296); its mutation to histidine in /Mactoglobulin C requires a change of only one DNA base if it is glutamine, but of 2 bases if it is glutamic acid. (296); (b) the peptide contains 4 serine residues to which a carbohydrate chain might be attached, but it also may contain the sequence -Asn-Ala-Ser-; such sequences are relatively rare (217) and are often the point of attachment of carbohydrate chains via an iV-glucosidic link from iV^-acetylglucosamine to the amide N of asparagine (436).…”

Studies on β-Lactoglobulins A, B, and C. II. Preparation of Modified Proteins by Treatment with Carboxypeptidase A*

“…From previous examination of this reaction, the modified proteins should be lacking 2 moles each of carboxyl-terminal isoleucine and penultimate histidine per molecule (36,000 mw). Polyacrylamide and agar-gel electrophoreses at pH 8.6 indicate the purity of the preparations and suggest incomplete removal of histidine with modified /3-lactoglobulin C. Examination of ultraviolet spectra and optical rotation data suggests that the modified proteins Dur ing the investigation of the carboxyl-terminal sequence of the genetic /3-lactoglobulin variants A, B, and C, using carboxypeptidase A, it was observed that the C-terminal isoleucine and penultimate histidine are released at a much more rapid rate than the subsequent amino acids (Kalan et al, 1965). Davie et al (1959) treated mixed /3-lactoglobulin from pooled milk with carboxypeptidase A and performed some preliminary experiments in which modified proteins lacking the Cterminal isoleucine, or histidine and isoleucine, were crystallized.…”

“…Since histidine is one of the pair of differing amino acids between /3-B1 and /3-C (Kalan et al, 1964(Kalan et al, , 1965 modified proteins containing two fewer histidine residues per molecule would be helpful in the search for the location of the amino acid substitution. Each /3-lactoglobulin molecule (36,000 mw) consists of two identical chains with molecular weights of 18,000 (Townend et al, 1960a).…”

“…These modified /3-lactoglobulins can also be utilized in the further examination of several structural properties such as the tetramerization mechanism as described by Townend et al (1960b), Townend and Timasheff (1960), and Timasheff and Townend (1961), the failure of the variants to hybridize (Townend et al, 1961), and the decreased rate of carboxypeptidase A hydrolysis demonstrated by /3-C (Kalan et al, 1965). retain a configuration similar to that of the native materials, and that the isoleucine and histidine aie located on the external surface of the molecule.…”