Lactose synthase system

Recently, the molten globule state of a-lactalbumin has been shown to possess antitumor activity when complexed with a fatty acid [36,37], and hence the protein may possess secondary biological activity in addition to the primary activity of native a-lactalbumin, i.e., substrate specificity modifier activity in a lactose synthase system [38,39]. The molten globule of a-lactalbumin thus provides an example of the folding intermediate of a protein exhibiting a secondary biological activity. 

The substrate specificity of the lactose synthase system was studied further by Brew et al. (1968). They confirmed that neither A protein nor B protein alone was active for the synthesis of lactose, but the A protein catalyzed the following reaction (see also Eig. 2) ... 

A necessary, but insufficient, property for a protein to be an a-lactalbumin is its ability to act as a specifier in the lactose synthase system. There is, at present, controversy as to whether a true a-lactalbumin occurs in the milk of monotremes. Thus, it is essential to have good methods for the determination of galactosyltransferase and lactose synthase activities. They must enable the detection of low levels of activity. This is... 

Musci and Berliner (1985b) concluded that apo-a-lactalbumin is more efficient as the modifier protein in the lactose synthase system than is the Ca(II)-bound form. They found that Vniax for the apo form shows a 3.5-fold increase over that for the Ca(II)-bound form, but there is no difference in (app.) between the two forms. They also confirmed that calcium stabilizes the protein against thermal denaturation (see Section IX,E), but that zinc is crucial in shifting the protein toward the apo-like form that is optimally active in lactose synthase. Their model is summarized schematically in Fig. 9. 

Many years ago Hopper and McKenzie (1974) noted structural similarities between equine and echidna lysozymes. They also obtained some evidence, albeit controversial, of a weak ability of echidna lysozyme to act as a modifier in the lactose synthase system. More recently, McKenzie and White (1987) noted very weak lytic activity in a variety of a-lactalbumin preparations. Also, Teahan et al. (1986, 1990) confirmed certain essential structural features for Ca(II) binding in echidna lysozymes I and II and noted the potential binding of Ca(II) by equine and pigeon lysozymes. D. C. Shaw and R. Tellam (quoted by Godovac-Zimmermann et al., 1987) made preliminary fluorometric observations that indicated binding of Ca(II) by echidna and equine lysozymes. 

Two variants of a-lactalbumin, caprine and ovine, have no Met residues, indicating that this residue plays no direct role in the lactose synthase system. Of the lysozymes only baboon milk and pigeon egg-white lysozymes have no Met residues. 

C. Interactions of Galactosyltransferase and a-LactaUmmin in the Lactose Synthase System... 

Much has been accomplished, especially in recent years, toward the goal of elucidating the active sites of galactosyltransferase and a-lactal-bumin. To this end, alteration of specific residues with observations of consequent effects on structure and activity is enlightening, as are metal ion effects. Where the substrate is concerned, we now have some detailed structural information for both galactosyltransferase and the lactose synthase system. 

It has been conventional wisdom that lysozyme is not active in the lactose synthase system and that a-lactalbumin does not have lytic activity. The essential residues for interaction of specifier protein with ga-lactosyltransferase have not yet been unequivocably defined, nor has the role of Ca(II) in this system. Thus, it is not, at present, possible to rule out weak specifier activity for lysozyme in the lactose synthase system. 

D-Galactosyltransferases.—The D-galactosyltransferase associated with the lactose synthase system in cows milk exists as a monomer (mol. wt. 4.6 x 10 ), although its aggregation is promoted by 2-acetamido-2-deoxy-D-glucose. Complexation of the enzyme with a-lactalbumin was investigated. 

Lactose Synthases.—The chemistry, biochemistry, and immunology of the lactose synthase system have been reviewed. ... 

Conformational changes that occur on immobilization of a-lactalbumins on agarose cyclic imidocarbonate have been studied by fluorometric methods. Immobilized bovine a-lactalbumin has been employed in a study of acceptor-ligand interactions in the lactose synthase system. ... 

Lactose Synthases.—Lactose synthases and related systems have been reviewed. Affinity chromatography has been used in a quantitative study of acceptor-ligand interactions in the lactose synthase system. The effects of D-glucose and 2-acetamido-2-deoxy-D-glucose on the elution of the A-protein of human lactose synthase from a column of a-lactalbumin immobilized on agarose cyclic imidocarbonate gave values of 200 and 0.571 mol , respectively, for the association constants representing the interaction between the enzyme and the monosaccharides. 

The reactions of bovine a-lactalbumin with JV-bromosuccinimide and 2-hydroxy-5-nitrobenzyl bromide have led to a better understanding of the interaction of a-lactalbumin with the lactose synthase system and of the conformation of a-lactalbumin. Comparison of the properties of denatured forms of bovine a-lactalbumin and of hen egg-white lysozyme showed that different regions of the molecules unfold in response to denaturants. Amino-acid analyses confirmed that the compositions of the backbones of the two molecules are similar. 

The tyrosyl residues of a-lactalbumin from human milk have been nitrated with tetranitromethane three components, which differ in the extent of nitration, were detected. The effect of nitration on the activity of a-lactalbumin in the lactose synthase system was investigated. 

Treatment of human a-lactalbumin with diethyl pyrocarbonate reduced its activity in the lactose synthase system, although incubation of the modified enzyme with hydroxylamine restored the activity. ... 

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