title: Involvement of a disulfide bridge in catalytic activity of camel lens xi-crystallin.
abstract: Chemical modification studies were performed to elucidate the role of cysteine residues in the catalytic activity of camel lens xi-crystallin. 5,5'-dithiobis (2-nitrobenzoic acid) (DTNB) titration of the camel lens xi-crystallin revealed that it contained 3 SH-groups and a disulfide bridge per subunit. One of the three SH-groups was readily accessible, whereas the other two were buried. Inactivation of xi-crystallin by DTNB was caused by a modification of one cysteine residue per subunit. The resulting DTNB-modified enzyme was reactivated by dithiothreitol (DTT) or KCN. The inactivation was partially protected by NADPH, whereas 9,10-phenanthrenequinone (PQ) enhanced the modification of the enzyme. These results indicate that the SH- group being modified is located at/near the NADPH binding site which is not essential for catalysis. Incubation of the enzyme with DTT led to a substantial loss of the enzyme activity. However, DTT inhibition was prevented completely by preincubation of enzyme with PQ but not by NADPH indicating that an essential disulfide-bridge is present at the substrate binding site of xi-crystallin.
authors.affiliation: Department of Biochemistry, College of Science, King Saud University, Riyadh, Saudi Arabia.
authors.forename: A S
authors.initials: AS
authors.lastname: Duhaiman
authors.forename: N
authors.initials: N
authors.lastname: Rabbani
mesh_terms: D000818:Animals; D002162:Camelus; D002384:Catalysis; D003459:Crystallins / Q000737:chemistry / Q000378:metabolism*; D004220:Disulfides / Q000378:metabolism*; D004228:Dithionitrobenzoic Acid; D004229:Dithiothreitol / Q000494:pharmacology; D004793:Enzyme Reactivators / Q000494:pharmacology; D007202:Indicators and Reagents; D007460:Iodoacetamide / Q000494:pharmacology; D009249:NADP / Q000378:metabolism; D013861:Thiocyanates / Q000494:pharmacology
chemical_list: D003459:Crystallins; D004220:Disulfides; D004793:Enzyme Reactivators; D007202:Indicators and Reagents; D013861:Thiocyanates; D009249:NADP; D004228:Dithionitrobenzoic Acid; D004229:Dithiothreitol; C009941:potassium thiocyanate; D007460:Iodoacetamide