A fatal variant of human ornithine carbamoyltransferase is stimulated by Mg2+.

Biochemical studies of a female who died at 2 years of age from a possible genetic variant of ornithine carbamoyltransferase (OCTase) deficiency are reported. The patient had severe psychomotor retardation with plasma ammonia levels throughout life reaching as high as 500 mumole/liter. The average OCTase level in the patient's liver was 2% of that in normal livers. Preincubation with 0.05 M MgCl2 resulted in a 570% increase in OCTase activity (13% of control). Citrate synthase and carbamoyl-phosphate synthase I were present at essentially normal levels. Unusual Mg2+ requirements have not been recognized in previous reports of OCTase deficiency, suggesting a genetic variant in this patient.