A systematic study of the metabolic fate of AMP, IMP, GMP and XMP (NMP) in the presence of cytosol from rat brain is here presented; the kinetics of both disappearance of NMP, and appearance of their degradation products was followed by HPLC. In the absence of ATP, AMP was preferentially degraded to adenosine with concomitant appearance of inosine and hypoxanthine. In the presence of ATP, AMP was preferentially degraded via IMP. The nucleosides generated in the course of the reactions are further degraded, almost exclusively, via nucleoside phosphorylase using as cofactor the P i generated in the reaction mixture. In order to quantify the effect of each one of the enzymes involved in the degradation of NMP, two complementary approaches were followed: (i) the V max and K m values of the enzymes acting in the intermediate steps of the reactions were determined; (ii) these data were introduced into differential equations describing the concentration of the nucleotides and their degradation products as a function of the time of incubation. Factors affecting kinetic parameters of the equation velocity as a function of ATP concentration were introduced when required. The differential equations were solved with the help of MATHEMATICA 3.0. The theoretical method can be used to simulate situations not feasible to be carried out, such as to measure the in¯uence of nM±mM concentrations of ATP on the metabolism of AMP. Keywords: adenylic charge, differential equations, purine salvage, quantitative metabolism J. Neurochem. (2001) 76, 1291±1307.5 H -Nucleotidases (EC 3.1.3.5) catalyse the hydrolysis of purine or pyrimidine nucleoside 5 H -phosphates to the corresponding nucleosides and inorganic phosphate. An ecto 5 H -nucleotidase (e-N) (glycosyl-phosphatidylinositol (GPI)-anchored) and three forms appearing in the cytosolic subcellular fraction (e-Ns, c-N-I, c-N-II) have been described in vertebrates (for a review see Zimmermann 1992). Only two of these enzymes have been described in the cytosol of rat brain: e-Ns and c-N-II. e-Ns appears to be derived from the glycosyl phosphatidyl inositol (GP-I) anchored e-N (Orford and Saggerson 1996); it has a preference for AMP (low K m ) and is inhibited by ATP and ADP (K i , mm) and by EDTA (Ipata 1968;Mallol and Bozal 1983;Orford and Saggerson 1996). c-N-II has a preference for IMP and GMP over AMP. The enzyme puri®ed from rat brain (Marques et al. 1998) has molecular and kinetic characteristics similar to the enzyme from other sources: it is activated by dinucleoside polyphosphates (K a , mm, Pinto et al. 1986;Itoh and Yamada 1990), decavanadate (K a , nm, Le Hir 1991), ATP and ADP (K a , mM, Van den Berghe et al. 1977a; Itoh 1981). The enzyme from rat brain is also activated by polyphosphates (K a , mm, Marques et al. 1998). Depending on the extraction and assay procedures one of the two cytosolic 5 H -nucleotidases may pass unnoticed. In our experience, both enzymes can be detected in the cytosolic extracts of rat brain if the proper experimental conditions are set up.T...