Contribution of different amine oxidases to the metabolism of dopamine in bovine retina.

The contribution of monoamine oxidase (MAO) A, MAO B and semicarbazide-sensitive amine oxidase (SSAO) to the metabolism of dopamine in the bovine retina was studied. These activities were present in the optic nerve, iris, choroid and bovine retina, but they were absent in the lens. SSAO activity towards dopamine was present in the choroid and the retina, but not in the iris or the optic nerve. The corresponding kinetic values for this substrate in the retina and the choroid showed higher affinity for MAO A (Km 271 and 197 microM, respectively) than for MAO B (Km 861 and 404 microM, respectively). This effect was counteracted by the higher Vmax value for MAO B resulting in the Vmax/Km ratio being similar for both cases. The absence of detectable SSAO activity towards dopamine in these last two tissues contrasts with the presence of that enzyme when benzylamine was studied as substrate. These results indicate that two different SSAO activities could be present in the bovine eye.