A Survey of Calpain Inhibitors

Abstract

Calpain is unique among the cysteine protease family of enzymes in that it combines thiol protease activity with calmodulin-like activity. Despite its wide spread distribution the exact physiological function(s) of calpain is yet to be deciphered. The enzyme is however, implicated in a number of pathophysiological conditions. Due to the potential of calpain as a therapeutic target a number of inhibitors have been described for the enzyme. In this article we have grouped calpain inhibitors into those derived from natural sources, and those derived from chemical synthesis. Additionally, an overview of functional groups that have been used as warheads of calpain inhibitors is presented along with a discussion of the structure activity relationship studies of the address region of peptidyl calpain inhibitors. Recent work in this area has led to a better understanding of the structural requirements for tight binding of inhibitors to the active site of calpain. A discussion of peptidomimetic calpain inhibitors, nonpeptide calpain inhibitors, and selectivity of some calpain inhibitors are also presented. The recent disclosure of the crystal structure of a nonpeptide calpain inhibitor bound to a hydrophobic pocket on the calcium-binding domain of calpain has opened the door to future development of potent cell permeable nonpeptide calpain inhibitors.

Calpastatin Exon 1B-Derived Peptide, a Selective Inhibitor of Calpain: Enhancing Cell Permeability by Conjugation with Penetratin

“…It seems probable that other domains of the calpain molecule participate in substrate recognition and are responsible for the highly selective binding of protein substrates and of the endogenous protein inhibitor calpastatin. This is underscored by the results of structure-activity relationship (SAR) studies (Donkor, 2000) indicating that affinity and selectivity of calpain for small peptide substrates and inhibitors are low when compared to other cysteine proteases. The shortest peptide binding to calpain with outstanding affinity and selectivity is the 27-mer derived from exon 1B of calpastatin domain 1 (CP1B).…”

“…The search for cell-permeable calpain inhibitors has been an ongoing process following the discovery of the enzyme over 30 years ago (Donkor, 2000). Nonselective calpain inhibitors such as EDTA and EGTA (Ca 2 + -chela- The ubiquitous calpains, µ-and m-calpain, have been implicated in essential physiological processes and various pathologies.…”

“…The conjugate was shown to efficiently penetrate into living LCLC 103H cells, since it prevents ionomycin-induced calpain activation at 200-fold lower concentration than the non-conjugated inhibitor and is able to reduce calpain-triggered apoptosis of these cells. Penetratinconjugated CP1B seems to be a promising alternative to the widely used cell-permeable peptide aldehydes tors) were followed by more potent and/or more selective calpain inhibitors isolated from natural sources or synthesized (reviewed in Wang and Yuen, 1997;Donkor, 2000). Most of them react covalently with the catalytic cysteine residue of calpain, but, with few exceptions, their selectivity for the calpains is low.…”

“…Moreover, the calpain inhibitor calpeptin (Z-Leu-Nle-CHO) was shown to inhibit protein tyrosine phosphatases as well (Schoenwaelder and Burridge, 1999). During the last decade much effort has been spent in the development of peptidic, peptidomimetic or non-peptide calpain inhibitors of improved selectivity and good bioavailability, but few of these inhibitors have been completely characterized in terms of selectivity or have been used extensively as tools in basic research (Donkor, 2000;Hernandez and Roush, 2002).…”

Inhibition of human μ-calpain by conformationally constrained calpastatin peptides

“…Of particular interest in this context is the bound conformation of the 27-mer peptide, CP1B-w1-27x (compound 1; see Figure 1A), derived from subdomain 1B of human calpastatin, because this peptide inhibits calpain almost as efficiently as the parent whole inhibitory domain 1 of the protein (Maki et al, 1989). Amongst the synthetic calpain inhibitors that have been developed so far (for reviews see Wells and Bihovsky, 1998;Donkor, 2000), CP1B-w1-27x (1) stands out due to its unequaled affinity/selectivity profile. Consequently, these unique properties have been utilized in cell biological studies.…”