Dipeptidyl peptidase IV was first described by Hopsu-Havu & Glenner (1966) and purified from kidney by Hopsu-Havu et al. (1968). Enzymes of this class (for a review, see McDonald et al., 1971) hydroylse dipeptides from the N-terminus of susceptible substrates. Barth et al. (1974) demonstrated that dipeptidyl peptidase IV exhibits two specificities, involving substrates of the types X-Pro-Y and X-Ala-Y, and that it is sensitive to inhibition by di-isopropyl phosphorofluoridate. The enzyme is one of a group of membrane-bound peptidases located in the microvilli of the proximal convoluted tubule (Booth & Kenny, 1974). Starting with pig kidney cortex, dipeptidyl peptidase IV was solubilized by autolysis at pH 3.8, and purified 300-fold by chromatography on columns of CM-cellulose, DEAEcellulose, Sepharose 6B and hydroxyapatite, each step being monitored by an assay using Gly-Pro-2-naphthylamide as substrate. The preparation moved as a single band on polyacrylamide-gel electrophoresis and appeared to be homogeneous in the analytical ultracentrifuge. When either the microvillus membrane (Booth & Kenny, 1976) or a portion of the solubilized extract at an early stage in the purification was labelled with di-isopropyl [32P]phosphorofluoridate, the only labelled protein was found to cochromatograph with the active enzyme.

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