Enzyme-linked immunosorbent assay of carbamoylphosphate synthetase I: plasma enzyme in rat experimental hepatitis and its clearance

Carbamoylphosphate synthetase I (CPS I), a urea cycle enzyme, is located almost exclusively in the mitochondria of hepatocytes. The enzyme is unique in that it constitutes about 2-6% of total liver protein and is composed of a large subunit of 160 kD. We developed a sensitive enzyme-linked immunosorbent assay (ELISA) for measurement of the enzyme in plasma using an antibody against the rat enzyme. In galactosamine-induced rat acute hepatitis, plasma concentration of CPS I that was 1-2 micrograms/ml blood before the treatment, increased up to 125 micrograms/ml blood in 24 h after the treatment and decreased to a near control level in 72 h. Plasma concentration of ornithine carbamoyl-transferase (OCT), another urea cycle enzyme, reached a maximum in 24 h and then decreased a little more rapidly than that of CPS I. On the other hand, alanine aminotransferase activity reached a maximum in 36 h and decreased to a normal level in 96 h. In immunoblot analysis, the native CPS I polypeptide of 160 kD and its fragments of 140 and 125 kD were detected 24-48 h after the treatment. When purified rat CPS I and bovine OCT were injected intravenously into rats, the enzymes disappeared from blood roughly exponentially with apparent half-lives of about 67 and 18 min, respectively. Development of an ELISA for human CPS I and determination of the serum enzyme in various liver diseases remain to be performed.