Ornithine carbamoyltransferase deficiency: coexistence of active and inactive forms of enzyme.

Kinetic and molecular properties of liver ornithine carbamoyltransferase from a female patient with the enzyme deficiency were studied. The enzyme activity in the patient was 9% of that in controls. Kinetic properties of the enzyme from the patient appeared to be identical to those of the control enzyme; apparent Km values for L-ornithine and carbamoyl-phosphate at pH 7.2 were 1.3 mmol/l and 7.9 mumol/l, respectively. Patient and control livers contained similar amounts of protein cross-reactive with antibody to the bovine enzyme. No difference in the subunit size was observed between the patient enzyme and the control enzyme. When extracts of control liver were analyzed by isoelectric focusing, a major peak of immunoreactive protein with a pI value of 7.3 and a minor peak with a pI value of 6.8 were observed, and both forms were enzymatically active. On the other hand, extracts of the patient's liver gave a major peak of immunoreactive protein with a pI value of 7.0 and a minor one with a pI of 6.8; the minor form was enzymatically active while the major one showed little or no activity. These results indicate that the patient's liver contained an inactive form of ornithine carbamoyltransferase in addition to an active form of the enzyme, and may reflect the X-linkage of the enzyme at the molecular level.