Brain cell membrane Na +,K +-ATPase modification following hypoxia in the guinea pig fetus

Abstract

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.