Calcium‐activated neutral proteinase (calpain) activity in C6 cell line: Compartmentation of μ and m calpain

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Abstract
Calcium‐activated neutral proteinase (calpain) activity was determined, including in cytosol and membrane fractions, in rat glioma C6 cell line. The μ and m forms of calpain were separated by DEAE and phenylsepharose column chromatography and with removal of the endogenous inhibitor calpastatin. C6 cells contained more mcalpain than the μ isoform. More than 70% of mcalpain activity was membrane‐associated and 20% was cytosolic. Isolated plasma membrane also contained 69% of the mcalpain activity. In contrast, approximately 80% of μcalpain activity was cytosolic and 16% was membranous. Half‐maximal activity for μ and mcalpain was obtained at 1 μM and 0.2 mM CaCl2, respectively. Trypsin dissociation of cells reduced activity. Triton X‐100 stimulated mcalpain activity of the whole homogenate and the membrane pellet but not of the cytosol. Activity of the myelin marker enzyme adenosine 2′3′‐cyclic nucleotide 3′‐phosphohydrolase (CNPase), was also found in C6 cells. The identification of calpain and CNPase in C6 cells is in keeping with an interpretation that C6 differentiation resembles, at least in part, that of the myelin‐forming oligodendroglial cells.