The isolated perfused rat brain preparation in the study of monoamine oxidase and benzylamine oxidase: Lack of selective brain perfusion

Monoamine oxidase (MAO) is present in brain blood vessels, and a different amine oxidase, benzylamine oxidase (BzAO), is claimed to exist in porcine cerebral vessels. The object of the present investigation was to evaluate these deaminating activities in the structurally intact brain, utilizing the isolated perfused rat brain preparation (IPRB). [¹⁴C]Benzylamine (10 μM), a substrate for both BzAO and MAO, was perfused via the internal carotid arteries for 5 min, and deaminated metabolites were measured. BzAO and MAO activities were distinguished by the use of the selective inhibitors semi-carbazide and pargyline. Both BzAO and MAO deaminated benzylamine (10 μm), but over 90 per cent of the total deaminated products resulted from BzAO. This was due to the much lower K_m, value of benzylamine for BzAO (2.8μM) than for MAO (169 μM). In vitro assays, however, revealed that the brain contained no measurable BzAO activity, whereas all other head structures (skull, mandible, skin, skeletal muscle, eyes, periocular tissues and tongue) contained BzAO activity. MAO was present both within and outside the brain. These results suggest that the IPRB preparation is not specific for brain perfusion. Experiments with technetium-labeled microspheres showed that, although the brain is preferentially perfused on a per gram basis, 52 per cent of the total perfusate passed through the skull and 13 per cent through extracranial structures. Only 35 per cent was specific for the brain. Other experiments showed that perfusion of the intact rat head produced greater deamination of benzylamine than when the skin and 70 per cent of the muscle were removed. Additionally, perfusion via the pterygopalatine arteries, to bypass the brain, resulted in increased deamination. It is concluded that the IPRB preparation is not specific for brain perfusion and that BzAO activity is present in all head structures other than the brain. The presence of BzAO in bone, skin, and muscle is consistent with suggestions for a physiological function of the enzyme in connective tissue.