Calpain Inhibitor-like Fraction in Rabbit Cerebrospinal Fluid is probably an Oligopeptide

Abstract

We have proved before that μ-calpain can be a promising autogenous agent to replace chymopapain in chemonucleolysis treatment. Even in the case of accidental intrathecal injection, the undesirable neural damage of calpain can be neutralized by normal cerebrospinal fluid (CSF). The aim of this study was to characterize this unknown calpain inhibitor-like component in the CSF of normal rabbits. A heat-stable low molecular weight fraction in the CSF was identified. It was not extracted by chloroform solution, but labile to protease digestion. It inhibited the calpain proteolysis of microtubule-associating protein-2 (MAP-2) and membrane structure proteins of platelets and PC12 cells. It can block the autolytic activation of native human erytluocyte μ-calpain, where the conversion of the large subunit of μ-calpain from the 80-kDa form to the 76-kDa form was obstructed, with accumulation of the intermediate 78-kDa form. A possible calpain inhibition mechanism was discussed. After purification by paper chromatography, gel filtration and reverse phase chromatography, the composition of the calpain inhibitor was investigated. An oligopeptide with molecular weight about 800 Da was identified as a novel calpain inhibitor by partial amino acid sequence.