High activity of semicarbazide-sensitive amine oxidase (SSAO): an important source of errors in the determination of the concentration of dopamine in pig plasma.

We noted rapid breakdown at 4 degrees and 20 degrees C of dopamine (DA) (but not of (nor)epinephrine and epinine) in pig plasma, but not in human plasma. The enzyme responsible appears to be a semicarbazide-sensitive amine oxidase (SSAO) because the breakdown can be inhibited by semicarbazide, but not by pargyline, clorgyline, EDTA, or (extra) glutathione. Among catecholamines tested, only DA and 3,4-dihydroxybenzylamine (DHBA, the internal standard of most catecholamine assays using high-performance liquid chromatography (HPLC) with electrochemical detection) were good substrates for the pig plasma SSAO. At 37 degrees C, especially after prolonged storage, all catecholamines break down. This breakdown results from autoxidation since it can be prevented by addition of extra glutathione (but not by semicarbazide) for all catecholamines except DA and DHBA. Breakdown at 37 degrees C of these two compounds cannot be prevented by addition of extra glutathione or semicarbazide, but only by addition of both. For reliable measurements of DA concentrations in pig plasma, blood should be collected in tubes containing not only glutathione, but also semicarbazide. The possibility of similarly high plasma SSAO activity in other species should be investigated further.