A novel mechanism-based inhibitor (6′-bromo-5′,6′-didehydro-6′-deoxy-6′-fluorohomoadenosine) that covalently modifies human placental S-adenosylhomocysteine hydrolase

Most inhibitors of S-adenosylhomocysteine (AdoHcy) hydrolase function as substrates for the "3'-oxidative activity" of the enzyme and convert the enzyme from its active form (NAD(+)) to its inactive form (NADH) (Liu, S,, Wolfe, M. S., and Borchardt, R. T. (1992) Antivir. Res. 19, 247-265), In this study, we describe the effects of a mechanism-based inhibitor, 6'-bromo-5',6'-didehydro-6'-deoxy-6'-fluorohomoadenosine (BDDFHA), which functions as a substrate for the "6'-hydrolytic activity" of the enzyme with subsequent formation of a covalent linkage with the enzyme. Incubation of human placental AdoHcy hydrolase with BDDFHA results in a maximum inactivation of 83% with the remaining enzyme activity exhibiting one-third of the k(cat) value of the native enzyme. This partial inactivation is concomitant with the release of both Br- and F- ions and the formation of adenine (Ade). The enzyme can be covalently labeled with [8-H-3]BDDFHA, resulting in a stoichiometry of 2 mol of BDDFHA/mol of the tetrameric enzyme. The H-3-labeled enzyme retains its original NAD(+)/NADH content. Tryptic digestion and subsequent protein sequencing of the [8-H-3]BDDFHA-labeled enzyme revealed that Arg(196) is the residue that is associated with the radiolabeled inhibitor. The partition ratio of the Ade formation (nonlethal event) to covalent acylation (lethal event) is approximately 1:1. From these experimental results, a possible mechanism by which BDDFHA inactivates AdoHcy hdyrolase is proposed: enzyme-mediated water addition at the C-6' position of BDDFHA followed by elimination of Br- ion results in the formation of homoAdo 6'-carboxyl fluoride (HACF). HACF then partitions in two ways: (a) attack by a proximal nucleophile (Arg196) to form an amide bond after expulsion of F- ion (lethal event) or (b) depurination to form Ade and hexose-derived 6-carboxyl fluoride (HDCF), which is further hydrolyzed to hexose-derived B-carboxylic acid (HDCA) and F- ion (nonlethal event).