BRAIN-CELL MEMBRANE NA+,K+-ATPASE MODIFICATION FOLLOWING HYPOXIA IN THE GUINEA-PIG FETUS

The effect of hypoxia in utero on the affinity of the active sites of Na+,K+-ATPase for Na+,K+ and ATP of the fetal guinea pig brain was investigated. Brain cell membranes were prepared from normoxic and hypoxic guinea pig fetuses, and a detailed enzyme kinetics analysis was carried out. In the hypoxic fetal brain membranes the K(a) 0.5 for Na+ and K+ increased 104% and 20%, respectively, indicating a decrease in the affinity of the active sites of the enzyme for Na+ and K+. The affinity of the ATP site increased, as indicated by a decrease in the K(m) of 37% in hypoxic brain. The results indicate that changes in the affinity of active sites would affect the phosphorylation and dephosphorylation mechanisms of the Na+, K+-ATPase reaction. The increased affinity for ATP will favor the phosphorylation step, but will be opposed by the decrease in the Na+ affinity. The decreased affinity of the active site for K+ would oppose the dephosphorylation of the enzyme-P complex causing the enzyme to be trapped in an inactive phosphorylated state. The results demonstrated the sensitivity of the Na+,K+-ATPase active sites to hypoxia, and illustrated a selective modification of the enzyme active sites under hypoxic conditions, a key mechanism altering the cell membrane function leading to hypoxic brain damage.