Triple Point Biologics' Cysteine Proteinase Inhibitors recombinant catalog covers seven members of the cystatin superfamily and the unrelated calpastatin family: cystatin A, B, C, D, S, and SA, together with calpastatin. Cystatins are tight-binding, reversible inhibitors of papain-family cysteine proteases (clan CA, family C1) and act by inserting a hairpin loop into the active-site cleft of target enzymes including cathepsins B, H, K, L, and S. Calpastatin is structurally distinct and functions as the endogenous inhibitor of the calcium-dependent calpain proteases (clan CA, family C2). Collectively, these inhibitors regulate intracellular and extracellular proteolysis across epithelial maintenance, lysosomal turnover, and calcium-dependent cytoskeletal remodeling. Cystatin C is secreted and freely filtered at the glomerulus, making it a widely used surrogate marker of glomerular filtration rate and a protein investigated in published studies of neurodegeneration and amyloid processing. Cystatin A is expressed predominantly in stratified squamous epithelium and has been characterized in studies of epidermal barrier function.

Each recombinant protein in this collection is expressed in HEK293 cells to preserve post-translational processing relevant to the mature, secreted or cytosolic forms, and is supplied as a carrier-free preparation with purity confirmed by SDS-PAGE and endotoxin testing. Every catalog entry is paired with a corresponding rabbit polyclonal antibody raised in Triple Point Biologics' own laboratory — the same program that has produced proteinase and inhibitor antibodies continuously since 1994. The matched antibodies are validated for Western blot and IHC; cross-reactivity with related family members is documented per product where experimentally determined.

These proteins are routinely used in matched-pair inhibition experiments alongside recombinant cathepsins to establish IC₅₀ values and define selectivity profiles — methodology detailed in the Triple Point Biologics Protease Activity Assays guide. Recombinant cystatins and calpastatin also serve as quantitative positive controls in Western blot and ELISA workflows; protocols for their use in this context are described in the Recombinant Protein Positive Controls guide. Lot-to-lot specific activity data are provided on each certificate of analysis to support experimental reproducibility across studies.