Multiple forms of Ca-activated protease from rat brain and muscle

Three Ca-dependent proteases have been identified in rat brain and skeletal muscle using ion exchange, gel filtration, and substrate affinity chromatography. A high degree of homology exists among three enzymes from different sources. Both the high molecular weight protease (154,000) and lower molecular weight protease (96,000) show high affinity for calcium while the third protease (76,000) had low affinity for calcium. Transformation among the three enzymes was calcium-induced and the process was unidirectional, generating a lower molecular weight form with decreased affinity for calcium. The protease with low affinity for calcium was susceptible to calcium-induced inactivation by autocatalysis. Immunologically the three proteases were equivalent, if not identical, and the brain and muscle proteases cross-react. All three proteases degraded neurofilament proteins; however, the protease with low affinity for calcium had 3 to 6 times higher specific activity. It is suggested that the high molecular weight enzyme (154,000) may be the native form of the Ca-dependent protease present in vivo.