Inosine 5'-monophosphate dehydrogenase (IMPDH) catalyzes the oxidation of inosine 5'-monophosphate (IMP) to xanthosine 5'-monophosphate with the concomitant reduction of NAD to NADH. Escherichia coli IMPDH is activated by K(+), Rb(+), NH(+)(4), and Cs(+). K(+) activation is inhibited by Li(+), Na(+), Ca(2+), and Mg(2+). This inhibition is competitive versus K(+) at high K(+) concentrations, noncompetitive versus IMP, and competitive versus NAD. Thus monovalent cation activation is linked to the NAD site. K(+) increases the rate constant for the pre-steady-state burst of NADH production, possibly by increasing the affinity of NAD. Three mutant IMPDHs have been identified which increase the value of K(m) for K(+): Asp13Ala, Asp50Ala, and Glu469Ala. In contrast to wild type, both Asp13Ala and Glu469Ala are activated by all cations tested. Thus these mutations eliminate cation selectivity. Both Asp13 and Glu469 appear to interact with the K(+) binding site identified in Chinese hamster IMPDH. Like wild-type IMPDH, K(+) activation of Asp50Ala is inhibited by Li(+), Na(+), Ca(2+), and Mg(2+). However, this inhibition is noncompetitive with respect to K(+) and competitive with respect to both IMP and NAD. Asp50 interacts with residues that form a rigid wall in the IMP site; disruption of this wall would be expected to decrease IMP binding, and the defect could propagate to the proposed K(+) site. Alternatively, this mutation could uncover a second monovalent cation binding site.