Protein Tolerance Tests Research Articles

ALLOPURINOL (AP) INDUCED OROTIDINURIA (ODNU): A TEST OF HETEROZYGOSITY FOR ORNITHINE TRANS-CARBAHYLASE (OTC) DEFICIENCY

In addition to its effects on purine metabolism, AP, via its metabolite, oxipurinol ribonucleotide, inhibits pyrimidine biosynthesis (PB) at the level of orotidine mono-phosphate decarboxylase. This inhibition results in increased ODNU. Women heterozygous at the OTC locus have 2 populations of hepatocytes; OTC normal and OTC deficient. We hypothesize that, as a consequence of carbamyl phosphate accumulation in the latter cells, PB is chronically increased. Therefore, inhibition of PB by AP should result in an even greater increase in ODNU in OTCH as compared to controls. The AP test includes administration of 300mg of AP followed by four pooled consecutive 6 hr urine collections. Urinary orotidine was measured by reverse phase HPLC after isolation of nucleosides on a boronate affinity column. We studied 11 control women (C) and 12 obligate OTC heterozygotes (OTCH). The mean (±SEM) peak urine orotidine (umol/mmol creatinine) of C and OTCH was respectively 7.36±0.46 (range 5.7-10.1) and 39.7±7.3. One OTCH fell in the normal range (7.71); other OTCH values ranged from 17.9 to 97.9. Comparison of these results with a protein tolerance test (PTT) administered to the same subjects, shows that the diagnostic sensitivity of the 2 tests are the same and that the same OTCH was negative in both. We conclude that PB is chronically increased in OTCH; and that the AP test identifies OTCH as well as the PTT, eliminates the risk of hyperammonemia, and is more convenient and acceptable.

803 IDENTIFICATION OF HETEROZYGOSITY FOR ORNITHINE TRANSCARBAMYLASE DEFICIENCY (OTCD)

This study was designed to determine the reliability of the protein tolerance test (PTT) in establishing heterozygosity for OTCD and hence aid in prenatal counseling and fetal diagnosis by DNA analysis in monoplex families. The results of the PTT done on 14 control women was compared to those done on 7 obligate heterozygotes. The peak urinary orotic acid (UOA) (ug orotic acid per mg creatinine) in the 14 control women varied from 0.4 to 2.0 with a mean (±SEM) peak value of 1.1. ± 0.11. The peak UOA in 7 obligate heterozygotes varied from 23.1 to 324 with a mean peak value of 179 ± 39. The absence of overlap and a p value of < .001 suggests that the PTT may be of great help in distinguishing between carriers and non-carriers of the OTCD gene. Accordingly, nine women at risk for being heterozygous for OTCD as a consequence of having borne an affected male or a symptomatic female were also studied: three of these women had UOA in the normal range (0.7, 0.98, 1.3); five had UOA in the obligate heterozygote range (20.9 to 73.5; one UOA value was ambiguous (4.9). Although these data require amplification they suggest that approximately one-third of male or female infants affected with OTCD are the result of a new mutation in a parental germ cell and that the mothers of these children are not heterozygotes. Women with ambiguous DOA values may be confirmed as carriers if a PTT done on the maternal grandmother of the affected child is unequivocally positive.