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Ammonia Detoxification And Urea CycleAmmonia Detoxification And Urea Cycle
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"In man, the urea cycle is the only known metabolic pathway of urea synthesis and the major one of ammonia detoxification. Genetic deficiencies of each of the urea cycle enzymes have been described with an overall prevalence estimated to be 1 in 30,000 live births. They are usually associated with hyperammonaemia, intolerance to protein ingestion and mental retardation."
Defects result in hyperammonaemic metabolic encephalopathy.
OTC deficiency is distinguished by extremely high levels of orotic acid in the urine formed when the excess carbamoyl phosphate accumulating in the mitochondrion leaks into the cytoplasm and is channelled into the pyrimidine biosynthetic pathway (Figure 1). Orotic acid levels may be normal when ammonia is well controlled. An allopurinol challenge is useful for the detection of carrier females. Such a test was performed on our patient. Initially, her urinary orotic acid level was high [11.8 mmol/mol creatinine (Cr), control <1.5] and it markedly increased to 50 mmol/molCr following allopurinol. In comparison, healthy individuals excrete 1–1.5 mmol/molCr and after allopurinol, their urine orotic acid level would increase to 3–8 mmol/molCr. Definitive diagnosis may be established by mutation analysis of the OTC gene.
Misdiagnosis and diagnostic delays are common.
In a patient with a history of previous encephalopathy, protein intolerance and mental deficit, OTC deficiency should be borne in mind.
ndt.oupjournals.org/cgi/content/full/17/7/1351
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