The inactivation kinetics of penicillin acylase from Escherichia coli have been investigated over a wide pH range at 25 and 50 degrees C. The enzyme was very stable in neutral solutions and quickly lost its catalytic activity in acidic and alkaline solutions. In all cases, the inactivation proceeded according to first order reaction kinetics. Analysis of the pH dependence of enzyme stability provides evidence that stable penicillin acylase conformation is maintained by salt bridges. Destruction of the salt bridges due to protonation/deprotonation of the amino acid residues forming these ion pairs causes inactivation by formation of the unstable "acidic" EH(4)(3+), EH(3)(2+), EH(2)(+) and "alkaline" E(-) enzyme forms. At temperatures above 35 degrees C penicillin acylase apparently undergoes a conformational change that is accompanied by destruction of one of these salt bridges and change in the catalytic properties.