The effect of modification and fragmentation of α-lactalbumin on lactose and lactosamine synthase reactions

Abstract

In the presence of the modifier protein α-lactalbumin, bovine milk galactosyltransferase transfers galactose to glucose forming lactose instead of transferring toN-acetylglucosamine formingN-acetyllactosamine. At low concentrations of α-lact-albumin, the lactosamine synthase activity is stimulated by α-lactalbumin and decreases when the lactose synthase activity develops along a sigmoidal curve. The observation suggests that different interactions between α-lactalbumin and enzyme were responsible for the modulating effect of the α-lactalbumin in the lactose and lactosamine synthase reactions.

To study the nature of the protein-protein interactions, α-lactalbumin was both modified and cleaved chemically. Reduction and alkylation with iodoacetic acid, iodoacetamide or 4-vinylpyridine abolished the ability of the α-lactalbumin to induce lactose synthase activity but stimulated lactosamine synthase activity 7-to 12-fold.

A peptide fragment corresponding to residues 26–60 of α-lactalbumin isolated from a 2-(2-nitrophenylsulphenyl)-3-methyl-3′-bromo-indolene (BNPS-skatole) fragmentation of the molecule was active in the lactosamine but not lactose synthase reaction. We concluded that, whereas lactose synthase required α-lactalbumin, in the native conformation, lactosamine synthase activity was stimulated by a linear sequence of amino acids in peptide 26–60.