Further characterization of phosphatase activities using non-specific substrates

The demonstration of non-lysosomal acid phosphatase has been the subject of a number of recent investigations. In the present study we compared the enzyme activities in rat liver and kidney that are revealed after incubation in the presence of either beta-glycerophosphate, p-nitrophenylphosphate or phenylphosphate at varying pH. As seen by others, the activity towards p-nitrophenylphosphate at pH 5-6 was confined to lysosomes, Golgi apparatus, endoplasmic reticulum (ER), nuclear envelope and plasmalemma. The reactivity of the plasmalemma and the ER was increased at pH 7. The TER of Küpffer cells in the liver stained intensely in contrast to the ER of the parenchymal cells, which stained only weakly. In the presence of NaF, all sites except the plasmalemma became negative. Addition of a levamisole-analogue, L-p-bromotetramisole, which is a specific inhibitor of alkaline phosphatase, resulted in the disappearance of the plasmalemmal activity whereas the activity at the other sites appeared unaltered. The rather unusual locations of activities with so-called non-specific substrates were further compared with those obtained with specific substrates such as glucose-6-phosphate and thiamine pyrophospate. The possible implication of these data in relation to the specificity of marker-enzymes for subcellular organelles is discussed.