Metabolic fate of AMP, IMP, GMP and XMP in the cytosol of rat brain: an experimental and theoretical analysis

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 Pi 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 influence of nM-muM concentrations of ATP on the metabolism of AMP.