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 Ka 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 Km 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.