Comparison of the structural properties of the active site cavities of human and rat monoamine oxidase A and B in their soluble and membrane-bound forms.

Structural properties of the active site cavities in human and rat monoamine oxidases (MAOA and MAOB) have been studied in their detergent-purified and outer mitochondrial membrane (OMM) bound forms using a spin-labeled irreversible inhibitor (ParSL) as an active specific spin probe. ParSL has been found to be 5-10-fold more specific for human MAOB (hMAOB) with a Ki of ca. 20 muM, compared to Ki's in the range of 100-200 muM observed for other human and rat MAOs. Solvent accessibilities of the active-site-bound spin probes have been determined by studying the power saturation properties of the spin probe EPR signals in the presence and absence of a polar paramagnetic reagent NiEDDA and by measuring the extent of spin probe reductions on treatment with excess ascorbic acid. Results presented here show that the spin probe bound to the hMAOA active site is ca. 7-8-fold more accessible than in hMAOB. In contrast, the spin probes covalently attached to the two rat enzyme active sites show comparable accessibilities to each other. On comparison of human versus rat enzymes, the active-site-bound spin probes in the two rat MAOs show ca. 40% less accessibilities compared to the same in hMAOA but ca. 4-5-fold higher accessibilities than in hMAOB active site. The present data thus suggests that the structural properties of the active site cavities in rat MAOs are significantly different compared to those in the two human enzymes, which correlates with the differences reported earlier in the inhibitor specificities between human and rat MAOs.