title: The mechanism of inactivation of human placental S-adenosylhomocysteine hydrolase by (E)-4',5'-didehydro-5'-methoxyadenosine and adenosine 5'-carboxaldehyde oxime.
abstract: The mechanisms by which (E)-4',5'-didehydro-5'-methoxyadenosine (DMOA) and adenosine 5'-carboxaldehyde oxime (ACAO) inactivate S-adenosylhomocysteine (AdoHcy) hydrolase were elucidated in this study. Their inhibitory activities toward AdoHcy hydrolase were found to be time- and concentration-dependent, and DMOA and ACAO had K(i) and k2 values of 3.0 microM and 0.10 min(-1) and 0.67 microM and 0.16 min(-1), respectively. The inactivation of AdoHcy hydrolase by DMOA (and ACAO) occurs concomitantly with the reduction of the enzyme-bound NAD+ to NADH. The rates of enzyme inactivation correspond to the rates of NADH formation. Incubation of both DMOA and ACAO with the NAD+ form of AdoHcy hydrolase resulted in formation of 3'-ketoadenosine (3'-keto-Ado) 5'-carboxaldehyde and its 4'-epimer. Incubation of DMOA and ACAO with the apo form of the enzyme afforded adenosine (Ado) 5'-carboxaldehyde and its 4'-epimer. These results show that DMOA and ACAO are "proinhibitors" of the enzyme. They are first converted to the inhibitors (Ado 5'-carboxaldehyde and its 4'-epimer) in the active site of the enzyme; these inhibitors then inactivate the enzyme by a type I mechanism. The results from this study demonstrated that this is a common mechanism by which 4',5'-didehydroadenosine analogs, serving as substrates of both the 5'-hydrolytic activity and the 3'-oxidative activity of the enzyme, inactivate AdoHcy hydrolase. The results also provide further evidence supporting the hypothesis that AdoHcy hydrolase possesses a 5'-hydrolytic activity independent of the 3'-oxidation activity.
authors.affiliation: Department of Biochemistry, University of Kansas, Lawrence 66045, USA.
authors.forename: H
authors.initials: H
authors.lastname: Huang
authors.forename: C S
authors.initials: CS
authors.lastname: Yuan
authors.forename: S F
authors.initials: SF
authors.lastname: Wnuk
authors.forename: M J
authors.initials: MJ
authors.lastname: Robins
authors.forename: R T
authors.initials: RT
authors.lastname: Borchardt
mesh_terms: D000241:Adenosine / Q000031:analogs & derivatives* / Q000494:pharmacology; D043383:Adenosylhomocysteinase; D002384:Catalysis; D002851:Chromatography, High Pressure Liquid; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D006801:Humans; D006867:Hydrolases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D009243:NAD / Q000737:chemistry; D010084:Oxidation-Reduction
chemical_list: C106825:4',5'-didehydro-5'-methoxyadenosine; D004791:Enzyme Inhibitors; C106824:adenosine 5'-carboxaldehyde oxime; D009243:NAD; D006867:Hydrolases; D043383:Adenosylhomocysteinase; D000241:Adenosine