J. Inher. Metab. Dis. 21 677 È 678 (1998) SSIEM and Kluwer Academic Publishers. Printed in the Netherlands ( CASE REPORT Adult-onset arginase deÐciency D. M. Cowley1*, F. G. Bowling1*, J. J. McGill1, J. van Dongen1 and D. Morris2 1 Mater Misericordiae Hospital, South Brisbane, Queensland ; 2 Queensland Health Pathology Services, Herston, Queensland, Australia * Correspondence : Department of Chemical Pathology, Mater Hospitals, South Brisbane Q4101, Australia (EC 3.5.3.1) is the last enzyme in the urea cycle. The enzyme is present in Arginase two forms. The cytosolic type I isoform, which is present in most tissues, predomi- nates in the liver. Red cells contain type I arginase. The type II mitochondrial isoform predominates in kidney. The enzyme is thought to a†ect arginine avail- ability for nitric oxide neurotransmitter synthesis and ornithine availability for both the urea cycle and the ornithine d-aminotransferase pathway. All cases of reported arginase deÐciency (McKusick 207800) have involved the liver type and several mutations have been characterized (Prasad et al 1997). The deÐciency presents in the neonatal period or early childhood, with a spastic para- paresis, seizures and psychomotor retardation. Cases have persistent hyper- argininaemia, with episodic elevations of ammonium. Red cell and liver arginase activity are absent. While some cases of late diagnosis have been reported, all reported individuals have been a†ected from an early age. The present case was an 18-year-old woman who had been well as a child with normal growth and development. She presented for investigation of collapse with sudden onset of a spastic diplegia. No sensory deÐcit was evident. She had mild, tender hepatomegaly. She gave a history of being unwell with episodic nausea and vomiting over the previous 6 months, and had experienced some degree of lower limb weakness over the previous 2 weeks. She had commenced the oral contraceptive 1 year earlier and was on no other medications. Family history revealed consanguinity, with her parents being second cousins. Her father had died of a presumed cerebrovascular event at 47 years of age. Liver studies showed reduced synthetic function with coagulopathy, albumin 25 g/L, and raised transaminases. Metabolic investigation showed a normal ammonium of 23 kmol/L, elevated plasma arginine (546 kmol/L ; normal 20 È130), homoarginine (25 kmol/L ; normal \3), glutamine (1050 kmol/L ; normal 400 È850), but repeatedly normal urinary arginine (6 kmol/L ; normal \25) with mild elevations of urinary ornithine (74 kmol/L ; normal \50) and lysine (82 kmol/L ; normal \70). CSF showed increased arginine (50 kmol/L ; normal 15 È18) and slightly increased homoarginine (3.1 mmol/L ; normal \3). Abetalipoproteinaemia, Wilson disease, porphyria, and vitamin deÐciencies were excluded. MRI of brain and spinal cord revealed no abnormality. The EEG 677