K+-p-Nitrophenylphosphatase Activity in Rat Brain and Liver

K+ -p-nitrophenylphosphatase (K + pNPPase) is the enzyme, which is considered to be involved in K + -dependent hydrolysis of the phosphoenzyme in the reaction cycle of Na + , K + -ATPase. The aim + pNPPase in homogenates of the rat brain and liver. We determined p-nitrophenylphosphatase (pNPPase) activity in the presence of various ion combinations (Mg 2+ +K + , Mg 2+ , K + ). We found a higher total pNPPase activity in the brain (0.8±0.079 nkat/mg protein) than in the liver (0.08±0.01 nkat/mg protein). Contrary to the liver, the main part of the total brain activity was K + -dependent. The activity of K + pNPPase was significantly higher in cerebral cortex homogenates (0.86±0.073 nkat/mg protein) in comparison to those of the whole brain (0.57±0.075 nkat/mg protein). The specific K + pNPPase activity was two times higher in the isolated pellet fraction (0.911±0.07 nkat/mg protein), rich in synaptosomes, compared to the whole brain homogenate (0.57±0.075 nkat/mg protein). Our results demonstrate the high activity of K + pNPPase in the brain tissue and its distribution mainly into the pellet fraction, what might indicate a possible role of K + pNPPase in specific structures of the brain, e.g. in synaptosomes.