Eukaryotic initiation factor 2α kinase and phosphatase activity during postischemic brain reperfusion

When ischemic brain is reperfused, there is in vulnerable neurons immediate inhibition of protein synthesis associated with a large increase in phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 [eIF2 alpha, phosphorylated form eIF2 alpha(P)]. We examined eIF2 alpha kinase and eIP2 alpha(P) phosphatase activity in brain homogenate postmitochondrial supernatants obtained from rats after 3 to 30 min of global brain ischemia (cardiac arrest), after 5 min of ischemia and 5 min of reperfusion (5R), and after 10 min of ischemia and 90 min reperfusion (90R), Because it has been suggested that PKR might be specifically responsible for producing eIF2 alpha(P) during reperfusion, we also examined in brain homogenates from wild-type and PKR0/0 C57BL/6J x 129/SV mice the effect of 5 min of ischemia and 5 min of reperfusion on eIF2 alpha(P). Cytosolic brain elF2 alpha(P) in the 5R and 90R rats was 18- and 23-fold that of nonischemic controls without any change in the rate of elF2 alpha(P) dephosphorylation. There was no change in eIF2 alpha kinase activity between 3 and 30 min of ischemia but an 85% decrease in the 5R group; the 90R group was similar to controls. In wild-type and PKR0/0 mice total eIF2 alpha was identical, and there was an identical le-fold increase in eIF2 alpha(P) at 5 min of reperfusion. Our observations contradict hypotheses that PKR activation, loss of eIF2 alpha(P) phosphatase activity, or any general increase in eIF2 alpha kinase activity are responsible for reperfusion-induced phosphorylation of eIF2 alpha, and we suggest that the mechanism may involve regulation of the availability of eIF2 alpha to a kinase. (C) 1999 Academic Press.