6-Aminopenicillanic acid. VI. Formation of 6-aminopenicillanic acid from penicillin by enzymic hydrolysis

Penicillin amidase activity, resulting in the hydrolysis of phenoxymethylpenicillin to give 6-aminopenicillanic acid, was found to be widespread among micro-organisms. Partial purification of the enzyme, which was essentially exocellular, was obtained by precipitation with ammonium sulphate, or acetone, followed by dialysis. The pH optimum with phenoxymethylpenicillin as substrate was found to be approximately 9.0 with maximum enzyme stability at approximately pH 10.0. Over a relatively short period of reaction time the optimum temperature was approximately 50 °C. The K m value with phenoxymethylpenicillin was found to be 4.0 mg/ml. Using as substrate a concentration of 1 % phenoxymethylpenicillin, a conversion to 6-aminopenicillanic acid of 56 % of theoretical was obtained in the enzyme reaction mixture. Isolation of pure 6-aminopenicillanic acid from this reaction mixture was obtained in 50 % yield. 6-aminopenicillanic acid markedly inhibited the enzyme activity at a concentration of 4.0 mg/ml. Phenoxyacetic acid at this concentration was without effect. At higher concentrations, incubation of 6-aminopenicillanic acid and phenoxyacetic acid with the enzyme resulted in the synthesis of phenoxymethylpenicillin. The pH optimum for enzyme activity in the direction of synthesis was 5.0 to 5.5 compared with approximately 9.0 for hydrolysis.

6-Aminopenicillanic acid. V. 6-Aminopenicillanic acid as a substrate for penicillinase and an inducer of penicillinase formation

The pH optimum for 6-aminopenicillanic acid hydrolysis by both staphylococcal and Bacillus cereus penicillinase is sharper and lower than for benzylpenicillin. With staphylococcal penicillinase the optimum is 5.6 compared with 6.5 to 7.0 for benzylpenicillin. Similarly, with B . cereus penicillinase the optimum is 5.5 compared with 7.0 for benzylpenicillin. The Michaelis constant for 6-aminopenicillanic acid with both types of penicillinase is relatively high compared with that for benzylpenicillin. With staphylococcal penicillinase the K m = 0.03 m compared with < 0.0003 for benzylpenicillin. With B . cereus penicillinase the K m = 0.003 m for 6-aminopenicillanic acid compared with 0.00006 m for benzylpenicillin. With staphylococcal penicillinase at the respective pH optima for 6-aminopenicillanic acid and benzylpenicillin the V max. for the two substrates is the same. With B . cereus penicillinase the V max. for 6-aminopenicillanic acid is slightly lower than the corresponding value for benzylpenicillin. 6-Aminopenicillanic acid induces the formation of staphylococcal penicillinase.