Stability of D-5,5-Dimethyl-A2-thiazoline-karboxylic Acid in Relation to Its Possible Occurrence As a Degradation Product of Penicillin by the Exocellular Dn-Carboxypeptidase-Transpeptidase from Streptomyces R61 and the Membrane-bound Dwcarboxypeptidase from Bacillus stearothermophilus*

The stability of o-5,5-dimethyl-A’-thiazoline-4-carboxylic acid has been studied under various conditions. In 10 mu cacodylate, pH 6.5, and at 55”C, o-5,5-dimethyld’-thiazoline-4-carboxylic acid (at concentrations lower than 1 mu) is hydrolyzed into N-formyl-n-penicillamine with a half-life of 3 to 4 min. On this basis, it is very unlikely that D-5,5dimethyl-AZ-thiazoline-4-carboxylic acid could be one of the end products resulting from the cleavage of benzylpenicillin by the on-carboxypeptidase of Bacillus stearothermophilus (as reported by Hammarstrom and Strominger (1976) J. Biol. Chem. 251, 7947-7949). In 3 mu phosphate, pH 7.5, and at 37°C o-5,5-dimethyl-AZ-thiazoline-4-carboxylic acid (at concentrations lower than 1 mu) has a half-life of 45 min. On the basis of kinetic experiments carried out under these conditions with phenoxymethylpenicillin and the oo-carboxypeptidase-transpeptidase of Streptomyces R61, it is concluded that the primary product which arises from the thiazolidine moiety of the antibiotic molecule and gives rise to N-formyl-o-penicillamine, has a half-life of 10 min, a value which is not compatible with the hypothesis that D5,5-dimethyl-AZ-thiazoline-4-carboxylic acid would be an intermediate involved in the fragmentation pathway.