Purification and characterization of lens specific calpain (Lp82) from bovine lens

Ubiquitous type m-calpain and lens specific Lp82 calpain were separated and partially purified from fetal bovine lens and the enzymatic characteristics were compared. Lens m-calpain required 200 μM calcium for 1/2 maximal activity, while Lp82 required 30 μM. Both types of calpains were inhibited by 0.1 mM E64, and 5 mM iodoacetamide, but not by 1 mM phenylmethylsulfonyl fluoride. Lp82 was insensitive to 1 μM calpastatin peptide while m-calpain was effectively inhibited. In the presence of calcium, m-calpain lost most of its activity within 2 hr, while Lp82 was continually active for 18 hr. Both calpains cleaved the natural substrates βA3 and αB crystallins in a similar manner. However, incubation of αA crystallin with m-calpain removed ten amino acid residues from its C-terminus, while incubation with Lp82 removed only five residues. The latter truncation product of αA was also found in vivo. These data suggested that Lp82 may have a more important role than m-calpain in modification of crystallins during lens maturation.