Alpha-mercaptoacrylic acid derivatives as novel selective calpain inhibitors.

Abstract

Calpain is a family of cytosolic cysteine proteases activated by calcium (1–5). There are two major isoforms, µ- and m-calpain, requiring low and high micromolar calcium for in vitro activity, respectively. Both µ- and m-calpain have a large catalytic subunit (80 kDa) and a small regulatory subunit (29 kDa). Both subunits contain EF-hand calcium-binding structures which control the proteolytic activity of the enzyme. Calpains are implicated in a number of pathological conditions, including stroke, myocardial ischemia and cataract, where intracellular calcium levels are elevated markedly (6). In cerebral ischemia, synaptic glutamate (excitotoxin) buildup leads to excessive activation of ionotropic N-methyl-D-aspartate NMDA- and AMPA/kainate-receptors (7–8). The resultant calcium influx triggers calpain activation. Calpain then degrades cytoskeletal proteins which leads to the loss of cell integrity and cell death (9). Several peptide calpain inhibitors (e.g., calpain inhibitor I (acetyl-Leu-Leu-Nle-H) and MDL28170 (carbobenzoxy-Val-Phe-H)) were found to have neuroprotective effects in various models of excitotoxicity and/or ischemia (10-12). However, these agents are nonselective and cross-inhibit other cysteine proteases. Thus, it is highly desirable to develop selective and potent calpain inhibitors.