The interaction of bovine milk galactosyltransferase with lipid and alpha-lactalbumin.
The activation of galactosyltransferase (UDPgalactose: N-acetyl-D-glucosaminyl-glycopeptide 4-beta-D-galactosyltransferase, EC 2.4.1.38) by alpha-lactalbumin has been studied at low concentrations of alpha-lactalbumin where the relationship is sigmoidal. The sigmoidal shape of the activation curve was eliminated by neutral lipids such as phosphatidylcholine and phosphatidylethanolamine, detergents such as Triton X-100 or by an aggregated form of alpha-lactalbumin generated by crosslinking alpha-lactalbumin with dithiobissuccinimidylpropionate. It is proposed that these different reagents present a hydrophobic surface to the enzyme which is necessary for lactose synthase activity. In competition experiments, large amounts of alpha-lactalbumin were able to displace lipid from the enzyme as suggested by the loss of the lipid-activating effect in the presence of an excess of alpha-lactalbumin. Optimal lactose synthase activity was obtained when the ratio of lipid/alpha-lactalbumin/enzyme was 60:6:1. The mechanism by which the lipid effect was obtained probably involved a phase transition in the enzyme which was detected as a sharp break in the Arrhenius curve. The presence of phosphatidylcholine abolished the break demonstrating that full activity of the enzyme required both alpha-lactalbumin and lipid.
D000818:Animals; D002417:Cattle; D003432:Cross-Linking Reagents / Q000494:pharmacology; D003902:Detergents / Q000494:pharmacology; D004789:Enzyme Activation; D005260:Female; D005700:Galactosyltransferases / Q000378:metabolism*; D007700:Kinetics; D007768:Lactalbumin / Q000494:pharmacology*; D008892:Milk / Q000201:enzymology; D010713:Phosphatidylcholines / Q000494:pharmacology; D010714:Phosphatidylethanolamines / Q000494:pharmacology; D010743:Phospholipids / Q000494:pharmacology*; D013388:Succinimides / Q000494:pharmacology; D006022:beta-N-Acetylglucosaminylglycopeptide beta-1,4-Galactosyltransferase / Q000378:metabolism*
D003432:Cross-Linking Reagents; D003902:Detergents; D010713:Phosphatidylcholines; D010714:Phosphatidylethanolamines; D010743:Phospholipids; D013388:Succinimides; D007768:Lactalbumin; D005700:Galactosyltransferases; D006022:beta-N-Acetylglucosaminylglycopeptide beta-1,4-Galactosyltransferase; C011240:dithiobis(succinimidylpropionate)