Orotidine 5'-monophosphate shows a (15)N isotope effect of 1.0036 at N-1 for decarboxylation catalyzed by orotidine 5'-monophosphate decarboxylase. Picolinic acid shows a (15)N isotope effect of 0.9955 for decarboxylation in ethylene glycol at 190 degrees C, while N-methyl picolinic acid shows a (15)N isotope effect of 1.0053 at 120 degrees C. The transition state for enzymatic decarboxylation of orotidine 5'-monophosphate resembles the transition state for N-methyl picolinic acid in that no bond order changes take place at N-1. This rules out enolization to give a quaternary nitrogen at N-1 in the enzymatic mechanism and suggests a carbanion intermediate stabilized by simple electrostatic interaction with Lys-93. The driving force for the reaction appears to be ground-state destabilization resulting from charge repulsion between the carboxyl of the substrate and Asp-91.