Orotic aciduria fibroblasts express a labile form of UMP synthase.
Orotic aciduria (type 1) results from a mutation in the gene for UMP synthase, a bifunctional protein containing the two enzyme activities which convert orotic acid and 5-phosphoribosyl-1-pyrophosphate to UMP and CO2. In fibroblasts from individuals with orotic aciduria, these two enzymatic activities are about 1% of normal but increase dramatically when deficient cells are grown in the presence of 6-azauridine. Using a polyclonal antiserum to sodium dodecyl sulfate-denatured, pure human UMP synthase, we show that fibroblasts from a patient with orotic aciduria have a low level of immunoreactive UMP synthase protein. Pulse-chase analysis reveals that the UMP synthase is degraded rapidly in the deficient cells. Growth of deficient cells in 6-azauridine leads to an increase in UMP synthase protein and its two enzymatic activities via a decreased rate of proteolytic degradation of UMP synthase. UMP synthase in extracts from deficient cells is more readily denatured by heat and is stabilized after growth of cells in 6-azauridine. These data suggest that the detrimental deficiency of this one patient results from a structurally altered UMP synthase that is probably present in low steady-state amounts due to proteolysis and that this labile protein can be stabilized against heat denaturation and proteolytic degradation by 6-aza-UMP.
Perry
M E
ME
Department of Biochemistry and Nutrition, School of Medicine, University of North Carolina, Chapel Hill 27599.
Jones
M E
ME
D001380:Azauridine / Q000494:pharmacology; D002262:Carboxy-Lyases / Q000235:genetics*; D002455:Cell Division / Q000187:drug effects; D002460:Cell Line; D002478:Cells, Cultured; D005347:Fibroblasts / Q000166:cytology / Q000187:drug effects / Q000201:enzymology; D006801:Humans; D007700:Kinetics; D009097:Multienzyme Complexes / Q000172:deficiency / Q000235:genetics* / Q000378:metabolism; D009962:Orotate Phosphoribosyltransferase / Q000172:deficiency / Q000235:genetics* / Q000378:metabolism; D009963:Orotic Acid / Q000652:urine*; D009964:Orotidine-5'-Phosphate Decarboxylase / Q000172:deficiency / Q000235:genetics* / Q000378:metabolism; D010430:Pentosyltransferases / Q000235:genetics*; D011686:Purine-Pyrimidine Metabolism, Inborn Errors / Q000201:enzymology* / Q000652:urine
D009097:Multienzyme Complexes; D009963:Orotic Acid; C027157:uridine 5'-monophosphate synthase; D001380:Azauridine; D010430:Pentosyltransferases; D009962:Orotate Phosphoribosyltransferase; D002262:Carboxy-Lyases; D009964:Orotidine-5'-Phosphate Decarboxylase