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 t he presence of cytosol from rat brain is here presented; the kinetics of bo th disappearance of NMP, and appearance of their degradation products was f ollowed by HPLC. In the absence of ATP, AMP was preferentially degraded to adenosine with concomitant appearance of inosine and hypoxanthine. In the p resence of ATP, AMP was preferentially degraded via IMP. The nucleosides ge nerated in the course of the reactions are further degraded, almost exclusi vely, via nucleoside phosphorylase using as cofactor the Pi generated in th e reaction mixture. In order to quantify the effect of each one of the enzy mes involved in the degradation of NMP, two complementary approaches were f ollowed: (i) the V-max and K-m values of the enzymes acting in the intermed iate steps of the reactions were determined; (ii) these data were introduce d into differential equations describing the concentration of the nucleotid es 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 equat ions were solved with the help of MATHEMATICA 3.0. The theoretical method c an be used to simulate situations not feasible to be carried out, such as t o measure the influence of nM-muM concentrations of ATP on the metabolism o f AMP.