Intracellular localization of low and high calcium-requiring forms of calpain*

Monospecific and discriminative antibodies against two distinct forms of calpain (Ca2+-dependent cysteine proteinase) were obtained. Immunological cross-reactivity between the heavy subunits of calpain I (low Ca2+-requiring form) and calpain II (high Ca2+-requiring form) were calculated to be 15 to 17%, and two steps of affinity chromatography was required to obtain discriminative antibodies. Using these antibodies, we have shown that a minor but reproducible amount of calpain I is associated with the membrane of human erthrocytes. Indirect immunofluorescent staining of cultured PK 15 cell revealed diffuse and finely granular cytoplasmic staining. Porcine kidney stained with either anti-calpain I IgG or anti-calpain II IgG yielded strong stainings in proximal and distal convoluted tubules, while glomerulus, macula densa and interstitial cells were not stained.