Laboratory Studies
Beyond the inherent problems in diagnosis of any urea cycle disorder, arginase deficiency is somewhat difficult to diagnose.
The typical crisis associated with hyperammonemia is rare, and random measurement of blood ammonia levels during periods of clinical stability is not helpful.
Arginine excretion in urine is not usually massively increased because of isozyme induction; however, a urinary amino acid excretion pattern can be observed. The excretion pattern is similar to that found in cystinuria, with increased arginine, ornithine, lysine, and, possibly, cystine. It can be observed because of competitive inhibition of dibasic amino acid reabsorption by elevated arginine in the renal proximal tubule.
Plasma arginine levels may not be greatly increased in cases of self-restriction of protein intake; therefore, even experienced clinicians may fail to diagnose the disease. Urine orotic acid is usually mildly increased. [10] Plasma ammonia levels may be mildly increased or normal.
When mild-to-moderate elevated plasma arginine levels are observed in association with developmental delay and spasticity, a red cell arginase assay is indicated for definitive biochemical diagnosis.
Imaging Studies
Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) findings in a relatively large group of patients with arginase deficiency have been reported recently. [11] As in the vast majority of inherited biochemical disorders, and the urea cycle defects specifically, the results are not in any way unique to the disease. Hence, MRI as a part of the initial diagnostic evaluation contributes little, if anything.
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Compounds that comprise the urea cycle are sequentially numbered, beginning with carbamyl phosphate (1). At this step, the first waste nitrogen is incorporated into the cycle; N-acetylglutamate exerts its regulatory control on the mediating enzyme, carbamoyl phosphate synthetase (CPS), in this step. Compound 2 is citrulline, the product of condensation between carbamyl phosphate (1) and ornithine (8); the mediating enzyme is ornithine transcarbamylase. Compound 3 is aspartic acid, which is combined with citrulline to form argininosuccinic acid (ASA) (4); the reaction is mediated by ASA synthetase. Compound 5 is fumaric acid generated in the reaction that converts ASA to arginine (6), which is mediated by ASA lyase.
