Regional distribution of brain calpastatin and of calpain II. Activity with casein and with endogenous brain protein substrates.

We examined the regional distribution in rat brain of calpain II, the calcium-activated neutral proteinase maximally active in the presence of 2mM Ca(2+), and of calpastatin, the endogenous inhibitor of the enzyme. A single-step chromatographic procedure was used to separate the constituents before determination. With [methyl-(14)C]?-casein as substrate, specific activity of calpain II was lowest in the cortex. The activity in the other areas tested was 10-50% higher, except pons-medulla > spinal cord > cerebellum > hypothalamus > striatum > hippocampus > cortex. When calpain II activity of the various areas was tested with endogenous brain protein substrates (the neurofilament proteins NF 200, 150 and 70 [NFT], glial fibrillary acidic protein [GFAP], desmin and actin), activity in each substrate was seen to be heterogenous, with a slightly different pattern of heterogeneity for each. The pons-medulla again was the highest in activity, but the cortex was usually not the lowest. Calpastatin was somewhat more evenly distributed in the various brain regions examined. Comparison of the enzyme activity of the crude supernatant with that in the purified fraction showed that at least 50% of the activity in the supernatant was inhibited by the calpastatin present. The regional differences in the substrate specificity of neutral proteolytic activity indicate that in vivo protein metabolism is influenced regionally by heterogeneity both in enzyme and in substrate distribution.