Similarity and dissimilarity in subunit structures of calpains I and II from various sources as demonstrated by immunological cross-reactivity.

The structural relationship between calpain I (low Ca2+-requiring) and calpain II (high Ca2+-requiring) molecules and their respective larger (80K) and smaller (30K) subunit proteins of several non-muscular tissues and cells was studied by testing immunological cross-reactivities. In addition to qualitative analyses by a conventional double immunodiffusion method, quantitative data were obtained, for the first time, by enzyme-linked immunosorbent assays using affinity-purified anticalpain I and anti-calpain II immunoglobulins. The enzyme sources included rat kidney, porcine kidney and erythrocytes, and human erythrocytes. It was concluded that the 30K subunits are immunologically almost indistinguishable between calpains, either I or II, not only from the same but also from different sources, while the 80K subunits of different origins are immunologically related to variable extents but always with discrimination between calpain I and calpain II. Similarity of the 30K subunit proteins and dissimilarity of the 80K counterparts were further substantiated by their chromatographic and electrophoretic behaviors.