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pubmed:abstractText |
Cell-bound isozymes of penicillinase are distinguished from extracellular enzyme by their capacity to bind deoxycholate and to elute with an apparent molecular weight of 45,000 on gel filtration in its presence. By methods that are unlikely to involve changes in primary structure, the cell-bound forms (both from the plasma membrane and from the periplasmic vesicles) can be converted to forms that are very similar if not identical to the exo-form (i.e., eluting with a molecular weight of 24,000 in the presence and absence of deoxycholate). In the case of plasma membrane penicillinase, addition of 25 per cent potassium phosphate at pH 9.0 leads to a 65 per cent conversion in 20 minutes at 30 degrees . Vesicle fraction penicillinase can be converted by pH 9.0 treatment alone. We suggest that the conversion involves a change from a hydrophobic to a hydrophilic conformational type, and that this is the crucial step for enzyme secretion in microorganisms. A model is presented to account for existing data in which we postulate that monomers of the newly synthesized penicillinase in an extended hydrophobic conformation are inserted into the membrane at special growing points where they may change to a hydrophilic exoform, or polymerize to the major plasma membrane type of penicillinase.
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