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The substrate-induced inactivation of beta-lactamase I from Bacillus cereus 569/H has been studied. Both the wild-type enzyme and mutants have been used. The kinetics follow a branched pathway of the type recently analysed [Waley (1991) Biochem. J. 279, 87-94]. The substrate cloxacillin (a penicillin) formed an acyl-enzyme (characterized by m.s.), and it was probably the instability of this intermediate that brought about inactivation. A disulphide bond was introduced into beta-lactamase I (the wild-type enzyme lacks this bond) by site-directed mutagenesis: Ala-77 and Ala-123 were replaced by cysteine. Spontaneous oxidation yielded the disulphide. The activity of this newly cross-linked enzyme was a little diminished, but the stability towards inactivation by cloxacillin was not increased. A second mutant of beta-lactamase I was studied: this mutant lacked the first 17 residues, i.e. the first alpha-helix. The mutant had reduced activity towards ordinary (non-inactivating) substrates and no hydrolysis of cloxacillin could be detected. These mutant enzymes were expressed in Bacillus subtilis, and were purified from the extracellular medium.