LP82-Calpain, also known as lens-specific calpain (LP=lens protein), is an intracellular, calcium-dependent cysteine protease. LP-85 is identical to LP-82 calpain, except for the insert that distinguishes LP-85. The calpains have papain-like activity, thus the -pain nomenclature. Unlike M-calpain and m-calpain, which are ubiquitously expressed, LP-82 calpain was originally identified in rat lens. The human sequence for LP-82 has a mutation producing a stop codon, and is not thought to be expressed. Like the other calpains, the endogenous calpain inhibitor, calpastatin, counters LP82. Because of the limited distribution and sequence differences, LP-82 calpain is thought to have different roles than the other calpain family members. The calpain family members consist of a common small subunit, and a large variable subunit. Domains in the large subunit include the amino terminal domain-I, the proteinase domain-II, domain-III, and the EF-hand domain-IV. RP1-LP82/85-Calpain recognizes latent and active LP82-calpain and LP-85 calpain. The latent large subunits are 85 Kd and 82 Kd, and the amino terminal truncation at activation yields an approximately 60- 62 Kd form. A cascade of smaller forms can be seen with further activation. RP1-LP82/85-Calpain works well for Western Blotting applications, and when used against the reduced protein identifies bands at 85 Kd, 82 Kd, 62 Kd, 60 Kd, and a series of further cleaved active forms. RP1-LP85-Calpain will also bind to the non-reduced protein, and thus has application for immunoprecipitation, immunohistochemistry, and ELISA. LP 82 and LP85 calpain are cellular proteins, thus cell lysates work well for Western blots. A recommended starting concentration for Western blots is 1:1000 when using colorimetric substrates such as BCIP/NBT, and 1:5000 for chemiluminescent substrates. Higher concentrations of antibody may be needed for samples from more distantly related species.

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