Properties of dipeptidyl peptidase I from rabbit (Oryctolagus cuniculus) lungs

Dipeptidyl peptidase 1 (DAP I; EC 3.4.14.1) was isolated from rabbit lungs and purified to apparent homogeneity. Similarly as the liver and spleen enzymes, it requires Cl- ions for its activity and is activated by thiol compounds. With Gly-Arg-2-naphthylamide as a substrate the broad pH optimum for DAP I activity ranged from 5.0 to 6.5; at pH 6.0-9.0 the enzyme catalysed the transamidation reaction. The Arrhenius plot for the DAP I activity at pH 5.0 was inflected at a point corresponding to 28 C. The enzyme was inhibited by the protease inhibitors, N alpha-tosyl-L-lysylchloromethane, N-tosyl-L-phenylalanylchloromethane, phenylmethylsulphonyl fluoride and antipain. Molecular weight of DAP I determined by sedimentation equilibrium in the presence of substrate was 128 000; in the absence of substrate two molecular forms of Mr of 135 000 and 91 600 were revealed. The Mr value determined on Sephadex G-200 was 154 000 +/- 7000. Molecular weight of the subunits determined by the two methods was about 21 000. Association of subunits over the pH range 4.6-6.0 was favoured by lowered temperature (7 C), presence of the substrate and increased enzyme concentration in the presence of the substrate. Only the forms of higher molecular weight, probably hexamers and octamers, exhibited enzymatic activity.