Determination of ornithine carbamoyl transferase (EC 2.1.3.3) activity in plasma is important for detection of liver diseases. The assay established in this paper has been made optimum. A blank is needed containing both substrates, carbamoyl phosphate and ornithine. We used a new colorimetric assay, based on a complex with a phosphoferric-antipyrine reagent and diacetyl monoxime, to measure the citrulline formed. The highly sensitive assay permits low activities to be determined accurately. Values for blood plasma from 425 supposedly healthy people, varied from 0 to 16 U/liter (95th percentile), and 27% of this population showed an activity of less than 2 µmol of citrulline formed per minute per liter, 2 U/liter being the limit of the method’s sensitivity.

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