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Cathepsin D is found in the cell in two forms, one a single polypeptide chain (Mr 44 000) and the other a non-covalent complex of two peptides of Mr 14 000 and 30 000. These correspond to the N-terminal and C-terminal regions of the single chain from which they originate. It has been shown that the two forms of the enzyme are closely similar in secondary-structure content, in aromatic amino acid environment and in denaturation behaviour. The two-chain enzyme has half the specific activity of the single-chain form. The denaturation and renaturation of the single-chain cathepsin D has now been studied by c.d., fluorescence and enzyme activity. Activity is lost irreversibly on unfolding, but the loss of backbone ellipticity and of folded aromatic environment is 75% reversible. The enzyme unfolds in two main stages, and the kinetics of these transitions indicate the existence of at least two intermediate forms between the native and the fully unfolded states. A further form of the enzyme exists in 0.5 M-guanidinium chloride. It is characterized by having an activity 40% greater than that of the native state. This increase is not reversed on removing the denaturant. The similarities between cathepsin D and pepsin are discussed.

Conformation, structure and activation of bovine cathepsin D. Unfolding and refolding studies