ABSTRACT
Membrane dipeptidase (MDP; EC 3.4.11.19) is a widely distributed mammalian cell-surface zinc metallopeptidase. The enzyme displays a versatile specificity, being able to hydrolyse dipeptides including unsaturated dipeptides (dehydropeptides) and those with a D-amino acid in the C-terminal position. MDP is involved in vivo in the renal metabolism of glutathione, and in the lungs the enzyme appears to play a role in the metabolism of the peptidyl leukotrienes. MDP is also the only known example of a mammalian P-lactamase, an activity which has led to the development of selective inhibitors of the enzyme and which may provide an insight into the mechanism of action of related bacterial mettallo-P-lactamases. MDP was the first mammalian peptidase identified as being anchored in the plasma membrane by a covalently attached glycosyl-phosphatidylinositol (GPI) moiety. The GPI anchor on MDP has been extensively characterized in terms of its hydrolysis by phospholipases and its recognition by specific antibodies. Recently the structure of the GPI anchor of porcine MDP has been determined and compared with the glycan core structure of human MDP. Thus, the availability of large quantities of purified GPI anchor and specific antibodies to it are proving useful tools in examining the role of GPI structures in processes such as cell signalling and endocytosis. The isolation of cDNAs encoding MDP has allowed molecular biological approaches to be used to investigate the structure and mechanism of action of the protein. Although MDP is a zinc metalloenzyme it does not contain any of the currently identified zinc binding motifs, and thus represents a novel class of zinc metalloprotease. Site-directed mutagenesis and expression are being used to identify residues important in zinc binding and catalytic activity, as well as in maintaining the quaternary structure of this disulphide-linked homodimer.
