Metabolic network of nucleosides in the brain.

Brain relies on circulating nucleosides, mainly synthesised de novo in the liver, for the synthesis of nucleotides, RNA, nuclear and mitochondrial DNA, coenzymes, and pyrimidine sugar- and lipid-conjugates. Essentially, the paths of nucleoside salvage in the brain include a two step conversion of inosine and guanosine to IMP and GMP, respectively, and a one step conversion of adenosine, uridine, and cytidine, to AMP, UMP, and CMP, respectively. With the exception of IMP, the other four nucleoside monophosphates are converted to their respective triphosphates via two successive phosphorylation steps. Brain ribonucleotide reductase converts nucleoside diphosphates to their deoxy counterparts. The delicate qualitative and quantitative balance of intracellular brain nucleoside triphosphates is maintained by the relative concentrations of circulating nucleosides, the specificity and the K(m) values of the transport systems and of cytosolic and mitochondrial nucleoside kinases and 5'-nucleotidases, and the relative rates of nucleoside triphosphate extracellular release. A cross talk between extra- and intra-cellular nucleoside metabolism exists, in which released nucleoside triphosphates, utilised as neuroactive signals, are catabolised by a membrane bound ectonucleotidase cascade system to their respective nucleosides, which are uptaken into brain cytosol, and converted back to nucleoside triphosphates by the salvage enzymes. Finally, phosphorolysis of brain nucleosides generates pentose phosphates, which are utilised for nucleoside interconversion, 5-phosphoribosyl-1-pyrophosphate synthesis, and energy repletion. This review focuses on these aspects of brain nucleoside metabolism, with the aim of giving a comprehensive picture of the metabolic network of nucleosides in normoxic conditions, with some hints on the derangements in anoxic/ischemic conditions.