Uridine urinary excretion

Urinary excretion of modified nncleosides, originating from tiansfer-RNA, may be nsed as a biomarker for tnmonrs and AIDS. Dudley et al. [54-57] reported method development for the analysis of urinary nucleosides by LC-MS. Initially, LC-MS conditions were optimized [54]. In positive-ion ESI-MS, detection limits were achieved ranging from 7 pmol for tubercidin to 110 pmol for uridine. Next, a comparison was made between GC-MS, LC-MS on an ion-trap instrament, and capillary LC-MS on a triple-quadmpole instmment [55]. These methods proved complementary rather than that just one could be selected as optimal. Therefore, in the next study [56], all three techniques were applied to identily the unexpected 5 -deoxycytidine in the urine of a patient suffering with head and neck cancer. In another study [57], they demonstrated the detection of dA, 1-methyl-dA, xanthosine, 7V-l-methyl-dG, 7V-2-methyl-dG, 7V-2,7V-2-dimethyl-dG, A-2,7V-2,A-7-trimethyl-dG, inosine, and 1-methylinosine in urine samples from various cancer patients. 

On a low protein diet, when the blood ammonia is considerably decreased, the urinary excretion of these compounds is also reduced. Even so some orotic acid and uracil is always found, although uridine may be completely absent from the urine (L6). 

Classical orotic aciduria is a rare autosomal recessive disorder which is characterized by retarded growth and excretion of large quantities of orotic acid in the urine [221,222]. The disease was described in 1959 as an inborn error of pyrimidine biosynthesis in patients with crystals of orotic acid in the urine [223]. The urinary excretion of orotic acid by these patients was 1.34 g per day in contrast to approximately 0.014 g per day excreted by normal individuals [222,224]. When the diet of patients was supplemented with uridine, clinical remission and a remarkable reduction in orotic acid excretion took place [221,225,226]. 

The reduction in urinary excretion of both compounds following uridine therapy reflects the utilization of uridine for the formation of UMP by the salvage pathway. A similar phenomenon was observed in hereditary orotic aciduria following uridine replacement therapy which bypasses the congenital enzyme defect (Chapter 5). The reversal of 6-azauridine-induced orotic aciduria by hydroxyurea, methotrexate and cyclophosphamide [251] (i.e. by the drugs affecting the synthesis of DNA without any effect on orotic acid synthesis) suggests that the control of pyrimidine synthesis de novo is linked to DNA synthesis. 

Trace amounts of uracil, uridine, thymine, and cytosine may also be found in human urine, along with pseudouridine, 5-methylcytosine, 3-methylcytosine, and 2 -0-methylcytidine (6, 7). Immediately following X-irradiation the urinary excretion of deoxycytidine and thymidine is markedly increased (8). 

Certain metabolites of the pyrimidine pathway are excreted in excess of the normal in inherited metabolic disorders of the urea cycle. They include orotic acid, uridine, and uracil. Of these substances, only uracil is a normal constituent of urine, the other two either being absent or present in very small amount. They are readily detected qualitatively as dark bands at the appropriate Rf values when a paper chromatogram of the urine is examined under ultraviolet light. They may be estimated by an ion exchange method similar to that for urinary pseudouridine (R13). 

Ornithine carbamoyltransferase (ornithine transcar-bamylase) (EC 2.1.3.3). Gross elevation of blood ammonia. Elevated glutamine in plasma and cerebrospinal fluid. Urea excretion low. Urinary orotic add increased. Uracil and uridine in urine. Gene for enzyme X-linked. Condition severe in boys (0-0.2% of normal enzyme activity in liver), who die in postnatal period (some cases of late onset have been reported). Girls have 5-10% normal liver enzyme activity. Some girls have died in late infancy or childhood, and others have survived with restricted protein intake. Abnormal EEC, mental retardation, brain atrophy and hepatomegaly. 

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