SUMMARY: Only 30-50 % of the cell-bound penicillinase of NRRL 569 is neutralized by antiserum prepared against the exo-penicillinase. The unneutralizable fraction is not decreased by cell disintegration which liberates a proportion of the cell-bound enzyme into solution. Absence of neutralization cannot therefore be explained by the existence of a mechanical barrier which prevents access of antibody; it has in fact been shown to be due to the presence of another type of penicillinase, not liberated from the cells, which is enzymically similar to, but immunologically and physico-chemically distinct from, the exo-enzyme. This cell-bound penicillinase has been separated from the exo-enzyme by fractional precipitation with ammonium sulphate. It is inducible by penicillin, like the exo-enzyme, and is present in cells of the constitutive mutant strain, 569/H.


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