The physical properties and the methods used for interconversion of three forms of cholesterol oxidase extracted from Nocardia rhodochrous by treatment with Triton X-100, trypsin or buffer alone provide evidence that these forms differ chiefly in the possession or absence of a hydrophobic anchor region connected by a trypsin-sensitive region. The hydrophobic domain normally integrates the enzyme into the cell membrane and confers amphipathic properties on the solubilized enzyme, causing adsorption to hydrophobic resins, aggregation when detergent is removed and formation of mixed micelles with detergent and cholesterol resulting in surface-dilution kinetic behaviour and activation by relatively high concentrations of water-miscible solvents. By contrast, only the enzymic fragment is extracted with trypsin and it behaves as a conventional soluble enzyme and does not aggregate or interact with hydrophobic resins, detergents or water-miscible solvents. As no phospholipid could be detected in the enzyme extracts, the detergent appears to act as a substitute for the cell-membrane lipids that would normally interact with the hydrophobic region. This cholesterol oxidase is an example of a prokaryotic enzyme possessing two closely associated catalytic functions, dehydrogenase and isomerase activities, and an anchoring function.

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