Metabolic fate of plasma diamine oxidase: evidence of isolated and perfused rat liver uptake.

After injection of an intravenous bolus of heparin (15,000 IU) in two groups of subjects, 10 normal volunteers and 6 subjects with external biliary drainage, blood and urine samples were collected; in the latter group bile samples were collected also. All samples were assayed for diamine oxidase (DAO). Persistently high values of this enzyme were found in plasma of both populations after heparin stimulation, while no increase in enzymatic activity was detected in bile and urine. In order to confirm and support the hepatic clearance of DAO, liver uptake of the enzyme derived from porcine kidney, human plasma and human placenta was studied by perfusion of isolated rat liver. Disappearance curves of the enzyme derived from three different sources showed a prompt liver uptake: activity decreased by about 50% in 10 min (endocytic uptake) and a slower but constant reduction during the remaining 110 min of perfusion was observed. These data suggest the hypothesis of liver metabolism of plasma DAO.