The Effect of Tyrosyl Modification on the Activity of α-Lactalbumin in the Lactose Synthetase Reaction

Metal-ion binding and the molecular conformational properties of alpha lactalbumin

Mammary galactosyltransferase and alpha lactalbumin are the two protein components of lactose synthase which catalyze the transfer of galactose from UDP-gal to glucose in the presence of divalent cations. Recent studies suggest that alpha lactalbumin may have a broader function in modifying cell surface carbohydrates in cell-cell interactions and cell differentiation. Since the discovery that alpha lactalbumin, like galactosyltransferase, is a metalloprotein, there has been a great deal of interest in the metal-binding properties of this protein and how these relate to the metal-ion requirements of the lactose synthase reaction. The recent availability of an X-ray crystal structure of alpha lactalbumin has provided further impetus for establishing the molecular determinants of its biological activity. This review is directed toward critically examining and integrating our present knowledge of the properties of this protein, particularly the relationship between metal-ion binding and conformational state, and how these might relate to its biological function.

Nitration of tyrosyl residues in human alpha-lactalbumin. Effect on lactose synthase specifier activity

Alpha-Lactalbumin isolated from human milk was reacted with tetranitromethane in molar excess of 8-32 mol/mol of tyrosine. After gel filtration on Sephadex G-75, followed by chromatographic fractionation using DEAE-Sephadex A-25, three main components were separated, which differed from one another in the extent of nitration. These protein fractions were found to contain, respectively, one and two nitrotyrosine residues, or two nitrotyrosine residues together with one nitrotryptophan. The lactose synthase specifier activity of each of these components was measured and compared with that of unsubstituted alpha-lactalbumin. Comparison of kinetic parameters showed the chemically modified proteins to be only slightly less active when tyrosines were the sole residues modified. In sharp contrast the additional nitration of a single tryptophan residue totally abolished the specifying activity of alpha-lactalbumin. Circular dichroism spectra of the tryptophan derivative revealed some structural alteration when compared with the other two and with the native protein. The conclusion could also be confirmed by using a double-immunodiffusion technique. After hydrolysis of the derivatives with thermolysin, it was possible to localize the substituted residues in the known sequence of human alpha-lactalbumin. Tyrosine-103 was found to be more easily nitrated than tyrosine-18. These two residues seem, therefore, to be on the outer surface of the molecule and more exposed than tyrosine-36 and tyrosine-50. Some precautions are indicated in the use of tetranitromethane as a nitrating agent on the basis of complex products observed in the nitration of the free amino acids tyrosine and tryptophan and their derivatives.

A fluorimetric study of the interactions of insolubilized human alpha-lactalbumin with galactosyl transferase (A-protein) and with anti-alpha-lactalbumin antibodies

Intrinsic as well as extrinsic fluorescence of an immobilized protein was used for the study of the interactions between alpha-lactalbumin-Sepharose and protein ligands. The fluorescence peak of the human alpha-lactalbumin-agarose conjugate was shifted to the blue and quenched in the presence of the galactosyl transferase (A-protein), indicating the probable formation of a complex between both proteins. The natural fluorescence of human alpha-lactalbumin bound to Sepharose was specifically quenched in presence of antihuman alpha-lactalbumin antibodies. This change in fluorescence appears to be due to binding of the antibodies to the immobilized antigen. Furthermore, the extrinsic fluorescence of a bound dye such as 2-p-toluidinylnaphthalene-6-sulfonate was used to confirm the existence of binding between antibodies and alpha-lactalbumin-agarose, and to obtain values for the association constant. A value of 5.6-10(+6) M(-1) for the binding constant was reported, which compares favorably with other data obtained by equilibrium dialysis.

Isolation, composition and functional properties of alpha-lactalbumins from several species

1. Galactosyltranferases were isolated from bovine and pig milk and alpha-labtalbumins from milks of cow, pig, dog, rabbit, baboon and chimpanzee. 2. Alpha-Lactalbumins were characterized by gel electrophoresis, amino acid composition, ultraviolet absorbance, and ability to promote synthesis of lactose by the galactosyltransferases. 3. The order of K-m values of the various alpha-lactalbumins was the same with both cow and pig galactosyltransferases and differed from the order predicted from taxonomical classification of the species. 4. A280/A260 abosrbance ratios were correlated with K-m values suggesting involvement of tyrosyl groups in the function of alpha-lactalbumin.

Fluorimetric study of conformational changes of various alpha-lactalbumins on agarose carriers

1. Various insoluble alpha-lactalbumins (bovine, bovine glyco-alpha-lactalbumin, human and human nitrated) have been prepared by coupling these proteins on to an agarose gel with use of cyanogen bromide. 2. Some intrinsic fluorescence properties, such as fluorescence maximum and pH dependence, were considered in order to study conformational changes of the alpha-lactalbumins covalently bound to an insoluble matrix. Examination of the pH-fluorescence profiles as well as the position of the maximum in the emission spectrum indicates that the Sepharose matrix does not appreciably modify the conformation of human and bovine glyco-alpha-lactalbumins. Some changes in the fluorescence spectrum (peak shifting towards longer wavelength) was observed for bovine alpha-lactalbumin and appeared to be due to alteration of the environment of the tryptophan side-chains in the protein upon coupling to the agarose gel. The emission spectrum of the insolubilized human nitrated alpha-lactalbumin indicates that the polypeptide chain of this protein gained some native conformation when covalently bound to the carrier. 3. The extrinsic fluorescence of a bound dye, such as 2-p-toluidinylnaphthalene-6-sulfonate, was used to study and to compare the hydrophobic sites on the surface of insoluble alpha-lactalbumins with the same proteins in solution. Considering the fluorescence properties of the protein with dye complexes it was found that both states of alpha-lactalbumins (insoluble and free in solution) bind the dye with similar association constants. However, the positions of the maxima in the emission spectra are all somewhat shifted towards longer wavelengths, suggesting that the dye binding site is located in a more polar environment when the proteins are bound to agarose. The human nitrated alpha-lactalbumin retains about equal possibility of binding this fluorescent dye. 4. As shown for bovine alpha-lactalbumin in solution, the binding of 2-p-toluidinylnaphthalene-6-sulfonate towards the various insoluble alpha-lactalbumins was not appreciably modified by the presence of various small compounds such as sugars and UDP, which are effectors in the lactose synthetase function.