The role of proteolytic enzymes in focal ischaemic brain damage

Although various neuroprotective and fibrinolytic drugs are currently under evaluation in the acute stages of ischaemic stroke, their therapeutic potential is likely to be limited by unwanted side effects and a narrow time window of opportunity for intervention. Proteolytic enzymes are involved in the catabolism of peptide neurotransmitters and structural cellular proteins in normal brain and have been implicated in the pathogenesis of neurodegenerative disorders. We hypothesised that activation of these enzymes might also play a crucial role in effecting ischaemic neuronal injury, thereby providing a potential site for therapeutic intervention in human stroke. Focal cerebral ischaemia was induced by thermocoagulation of the left middle cerebral artery in aged (30 month) male Wistar rats who were pre-treated with saline or the competitive N-methyl-D-Aspartate antagonist D-CPP-ene, which has been shown to be neuroprotective in young animal models of stroke. Major protease activities were analysed in the left (ischaemic) and right (non-ischaemic) hemispheres, following tissue homogenisation. Data have been analysed using Mann-Whitney tests and are presented as means +/- standard errors. Enzyme activity decreased in ischaemic brain; for example, the mean activity of dipeptidyl aminopeptidase I was 23 +/- 3 and 43 +/- 6 nmol substrate/hour/ml brain extract in the left and right hemispheres respectively (n = 10, p < 0.05). Ischaemic neuronal injury is not effected by the early activation of proteolytic enzymes and protease inhibitors are therefore unlikely to be of benefit in human stroke.