CHARACTERIZATION OF THE MALTASE ACTIVITY OF GLUCOSIDASE-II FROM RAT-LIVER - KINETIC-MODEL

Glucosidase II is a key enzyme in the processing of N-glycoproteins since it removes the two glucose residues from the protein-linked oligosaccharide GIC2Man9GlcNAC2-R. We have studied the kinetics of the purified enzyme, using maltose as substrate. Analysis of data fitting to single and double-hyperbolic equations and the Eadie-Hofstee profile indicate that the enzyme has two binding (active) sites for the hydrolysis of maltose. The K(m) and V(max) values for the high-affinity site were 0.43mM and 691 mU/mg, respectively, whereas the values for the low-affinity site were 57.7mm and 2888 mU/mg, respectively. The V(max)/K(m) ratios were 1607 and 50.1 ml/min per g for the high- and low-affinity sites, respectively. A new kinetic model for this enzyme is proposed from the equilibria corresponding to the partial competitive inhibition produced by maltose on p-nitrophenylglucosidase activity. The amino acid composition of the enzyme has been established.