Catalyzes efficient racemization of N-succinyl-L-Arg and N-succinyl-L-Lys, suggesting that these are physiological substrates of this enzyme. Has low activity with L-Asp-L-Lys, and even lower activity with L-Leu-L-Arg, L-Leu-L-Lys, N-succinyl-L-His and N-succinyl-L-Met (in vitro). Binds 1 Mg(2+) ion per subunit. Part of a large, functionally divergent protein family. Protein modeling and substrate docking was used to predict the substrate specificity, prior to biochemical analysis. Belongs to the mandelate racemase/muconate lactonizing enzyme family.