Lactose synthase system lysozyme

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. 

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. 

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. 

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