Purine nucleoside phosphorylase (PNP) is an intracellular catalytic enzyme in the purine salvage pathway. Its deficiency is associated with T-cell lymphopenia, severe immunodeficiency and neurologic symptoms and delay in the brain development and its histochemical localization in the brain indicated that it was restricted to glial cells (Castellano et al, J Hist Cytochem; 1990). Furthermore, a PNP activity was observed in plasma as well as in cerebrospinal fluid, even though the source of such extracellular enzyme is substantially unknown.

The aim of this study was to assess whether cultures of astrocytes and microglial cells were able to release PNP into the culture medium. PNP is remarkably released from the two types of glial cells and microglia is more efficient in the enzyme secretion compared to astrocytes. Microglial activation induced by LPS or IFN-? did not cause any over-basal secretion whereas the stimulation with ATP or Benzoyl-ATP caused a dose-dependent increase in the release by glial cells.

The immunocytochemical analysis of PNP on astrocytes and microglial cells demonstrated that the enzyme was mainly localized into the cytosol, however it was frequently found also a granular staining that co-localized with LAMP-1, a typical lysosomal marker. This co-localization was more evident following cell challenge with high ATP concentrations or with Benzoyl-ATP.