Subcellular localization of dihydropyrimidine dehydrogenase

Evolutionary implications of the mosaic pyrimidine-biosynthetic pathway in eukaryotes

The de-novo pyrimidine biosynthetic pathway involves six enzymes, in order from the first to the sixth step, carbamoyl-phosphate synthetase II (CPS II) comprising glutamine amidotransferase (GAT) and carbamoyl-phosphate synthetase (CPS) domains or subunits, aspartate carbamoyltransferase (ACT), dihydroorotase (DHO), dihydroorotate dehydrogenase (DHOD), orotate phosphoribosyltransferase (OPRT), and orotidine-5'-monophosphate decarboxylase (OMPDC). In contrast with reports on molecular evolution of the individual enzymes, we attempted to draw an evolutionary picture of the whole pathway using the protein phylogeny. We demonstrate highly mosaic organizations of the pyrimidine biosynthetic pathway in eukaryotes. During evolution of the eukaryotic pathway, plants and fungi (or their ancestors) in particular may have secondarily acquired the characteristic enzymes. This is consistent with the fact that the organization of plant enzymes is highly chimeric: (1) two subunits of CPS II, GAT and CPS, cluster with a clade including cyanobacteria and red algal chloroplasts, (2) ACT not with a cyanobacterium, Synechocystis spp., irrespective of its putative signal sequence targeting into chloroplasts, and (3) DHO with a clade of proteobacteria. In fungi, DHO and OPRT cluster respectively with the corresponding proteobacterial counterparts. The phylogenetic analyses of DHOD and OMPDC also support the implications of the mosaic pyrimidine biosynthetic pathway in eukaryotes. The potential importance of the horizontal gene transfer(s) and endosymbiosis in establishing the mosaic pathway is discussed.

Profound variation in dihydropyrimidine dehydrogenase activity in human blood cells: major implications for the detection of partly deficient patients

Dihydropyrimidine dehydrogenase (DPD) is responsible for the breakdown of the widely used antineoplastic agent 5-fluorouracil (5FU), thereby limiting the efficacy of the therapy. To identify patients suffering from a complete or partial DPD deficiency, the activity of DPD is usually determined in peripheral blood mononuclear cells (PBM cells). In this study, we demonstrated that the highest activity of DPD was found in monocytes followed by that of lymphocytes, granulocytes and platelets, whereas no significant activity of DPD could be detected in erythrocytes. The activity of DPD in PBM cells proved to be intermediate compared with the DPD activity observed in monocytes and lymphocytes. The mean percentage of monocytes in the PBM cells obtained from cancer patients proved to be significantly higher than that observed in PBM cells obtained from healthy volunteers. Moreover, a profound positive correlation was observed between the DPD activity of PBM cells and the percentage of monocytes, thus introducing a large inter- and intrapatient variability in the activity of DPD and hindering the detection of patients with a partial DPD deficiency.