Principle Of Methods: Citrulline produced in the reactions is determined colorimetrically by its reaction with diacetylmonoxime. For the CPS reaction exogenous OTC is added to convert the carbamoyl phosphate formed to citrulline.
Uses And Limitations Of The Methods: The assays are carried out to diagnose the urea cycle defects, carbamoyl phosphate synthetase deficiency and ornithine transcarbamylase deficiency. Both enzymes are assayed in the same specimen, each serving as reference enzyme for the other. Liver (biopsy or postmortem) is the tissue used in the SAS laboratory, although rectal or duodenal biopsies have been reported suitable. Diagnosis is usually by exclusion of other urea cycle disorders until enzyme analysis is carried out. In addition to hyperammonaemia there is orotic aciduria, although not in pre-symptomatic later-onset patients. The SAS laboratory does not offer prenatal diagnosis for either disorder, although this has been achieved by fetal liver biopsy or DNA analysis. The laboratory has limited experience of diagnosing affected heterozygotes for OTC deficiency by enzyme analysis. Testing for heterozygotes by measuring orotidine excretion after an allopurinol load is an alternative method and is available in the Purine Research Laboratory at Guy’s Hospital.
Specimen Requirements: Liver biopsy (5-10 mg) should be snap-frozen in liquid nitrogen and transported to the laboratory in liquid nitrogen or solid CO2.
Advance Notice Of Arrival Of These Specimens Is Required.
The Laboratory Recommends Use Of A Courier Service Or Royal Mail Special Delivery For Sending All Specimens To The Laboratory.