Abstract

The hydrolytic action of lysozyme (muramidase) was inhibited 50% by benzylpenicillin (24 mM) in 0.1 M phosphate buffer at pH 7.0. 7-(Thiophene-2-acetamido)cephalosporin (cephalothin) exerted a similar inhibitory action.

Modification of some of the functional groups of these molecules, with a resultant decrease in their structural similarity to N-acetylmuramic acid, destroyed their inhibitory activity. Integrity of the lactam ring was not essential for activity.

Fluorescence measurements and ultraviolet difference spectroscopy indicated the formation of an enzyme-inhibitor complex. Penicillin and N-acetylglucosamine produced a perturbation peak at 293 mµ when mixed with the enzyme. N-Bromosuccinimide oxidation of the tryptophan residue in position 62, one of six tryptophan units in the enzyme molecule, destroyed enzymatic activity. This modification reduced the size of the perturbation peak induced by penicillin and eliminated the peak caused by N-acetylglucosamine.

The experimental results agree with previous indications that penicillin bears a structural resemblance to a portion of the bacterial cell wall, N-acetylmuramic acid.

In these studies, lysozyme has been used as a model for demonstrating the interaction between penicillin and an enzyme protein. The concentrations of penicillin required for inhibition of lysozyme are far greater than the level of the drug required for the inhibition of sensitive microorganisms.