The pyruvoyl-dependent histidine decarboxylase from Lactobacillus 30a is rapidly inactivated by incubation with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and glycine ethyl ester. On 90% of inactivation, 1.3 residues of [14C]glycine ethyl ester are incorporated per alpha subunit; nearly 60% of this is linked to the beta-carboxyl group of Asp-191. Histamine, a competitive inhibitor, protects against this inactivation. The KM value of the modified enzyme for histidine (6.2 mM) is much higher than that of the unmodified enzyme (KM = 0.4 mM); catalytic activity is reduced but not eliminated. Thus, Asp-191 is the most reactive accessible carboxyl group under these conditions and is close to the substrate-binding site, but apparently is not essential for catalysis. At pH 8.0, fluorodinitrobenzene inactivates histidine decarboxylase completely with the incorporation of two dinitrophenyl residues/alpha subunit; the modified residues are Lys-155 and Cys-228. Urocanic acid, a competitive inhibitor, protects against inactivation. Treatment with mercaptoethanol restores the free -SH of Cys-228 but does not restore activity. Conversion of Cys-228 to its cyano derivative slows but does not prevent dinitrophenylation of Lys-155; the resulting derivative is catalytically inactive. Thus, Lys-155 is located within the active site and may play an essential role in catalysis. Finally, histidine methyl ester was shown to inhibit this decarboxylase by forming a Schiff's base with the essential pyruvoyl group.