The ARH and Macrodomain Families of α-ADP-ribose-acceptor Hydrolases Catalyze α-NAD Hydrolysis.

ADP-ribosyltransferases transfer ADP-ribose from β-NAD to acceptors; ADP-ribosylated acceptors are cleaved by ADP-ribosyl-acceptor hydrolases (ARHs) and proteins containing ADP-ribose-binding modules termed macrodomains. On the basis of the ADP-ribosyl-arginine hydrolase 1 (ARH1) stereospecific hydrolysis of α-ADP-ribosyl-arginine and the hypothesis that α-NAD is generated as a side product of β-NAD/ NADH metabolism, we proposed that α-NAD was a substrate of ARHs and macrodomain proteins. Here, we report that ARH1, ARH3, and macrodomain proteins (i.e., MacroD1, MacroD2, C6orf130 (TARG1), Af1521, hydrolyzed α-NAD but not β-NAD. ARH3 had the highest α-NADase specific activity. The ARH and macrodomain protein families, in stereospecific reactions, cleave ADP-ribose linkages to N- or O- containing functional groups; anomerization of α- to β-forms (e.g., α-ADP-ribosyl-arginine to β-ADP-ribose- (arginine) protein) may explain partial hydrolysis of ADP-ribosylated acceptors with an increase in content of ADP-ribosylated substrates. Af1521 and ARH3 crystal structures with bound ADP-ribose revealed similar ADP-ribose-binding pockets with the catalytic residues of the ARH and macrodomain protein families in the N-terminal helix and loop. Although the biological roles of the ARHs and macrodomain proteins differ, they share enzymatic and structural properties that may regulate metabolites such as α-NAD.