J9 tli orphan

a sulphonic acid instead of a carboxy group e.g. m-aminobenzenesulphonic derivatives show a high degree of inhibitory potency and selectivity for MEP.

Hydrogen bond doner group. This binds with the terminal amide (peptide) linkage. Evidence that the amide group of the peptide bond between AA1 and AA2 is hydrogen bonded to the active site of the enzyme comes from the observation that N-methylation of the peptide link in the dipeptides Phe-Gly, Phe-Ala or Phe-Leu, leads to 100-fold reduction in inhibitory activity.

The amino acid sequence of MEP has now also been determined. It consists of 749 amino acids spanning the cell membrane and includes a 27-amino acid residue cytoplasmic domain, a 13-amino acid residue hydrophobic domain and a large extracellular domain containing the active site. The three zinc-coordinating residues have been identified as His-583, His-587 and Glu-646 and Glu-584 as the residue involved in the acid-base catalytic mechanism occuring at the active site.

The use of the sulphydryl group as the zinc-binding ligand inserted into dipeptides was shown to be optimal for binding with the active site and led to the development of the first potent inhibitor of MEP, thiorphan (8.59, Ki=4.7 nM). Thiorphan was also found to be a relatively efficient inhibitor of ACE (Ki=150 nM), retroinversion of the amide bond (retrothiorphan) increased selectivity for MEP (MEP, Ki=6-10 nM, ACE Ki>10 pM). The A-Carboxyalkyl-based MEP inhibitors SCH 32615 (8.60) and SCH 39370 (8.61) were developed from concepts similar to those used in the development of A-carboxyalkyl ACE inhibitors. Two aromatic amino acid residues occupying the S1 and S1' subsites combined with P-alanine or y-aminobutyric acid at AA2 enhanced MEP inhibitory potency and selectivity over ACE. It has been proposed that the A-carboxyalkyl group serves to bind the zinc and the P-alanine residue is a critical component in determining selectivity for MEP as significant ACE inhibitory activity is observed when alanine is present as AA2. More conformationally constrained molecules, based on y-aminobutyric acid in the AA2 position combined with cycloleucine at AA1 led to the development of

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