title: The sulfhydryl group microenvironment of lactose synthase from bovine milk.
abstract: Galactosyltransferase from bovine milk was inactivated by a series of sulfhydryl group specific reagents of different structures and sizes. The inactivation rate constants suggest that the thiol is located in a nonpolar microenvironment. The ESR spectrum of a spin labeled galactosyltransferase showed that the sulfhydryl group is in a region of non-restricted rotation, consistent with its broad reactivity towards various thiol reagents. Galactosyltransferase immobilized onto agarose through its sulfhydryl group retained its ability to catalyze the synthesis of N-acetyllactosamine and lactose. Thus the residual activity of the sulfhydryl group modified enzyme is not due to an isozyme lacking such a group. In addition, the active thiol can not be located at the active site nor the protein-protein interaction site between galactosyltransferase and alpha-lactalbumin.
mesh_terms: D000818:Animals; D001665:Binding Sites; D002417:Cattle; D004578:Electron Spin Resonance Spectroscopy; D005260:Female; D007700:Kinetics; D007788:Lactose Synthase / Q000037:antagonists & inhibitors*; D008892:Milk / Q000201:enzymology*; D011485:Protein Binding; D013329:Structure-Activity Relationship; D013439:Sulfhydryl Reagents / Q000494:pharmacology*
chemical_list: D013439:Sulfhydryl Reagents; D007788:Lactose Synthase