Demonstration of isoleucine 199 as a structural determinant for the selective inhibition of human monoamine oxidase B by specific reversible inhibitors

Several reversible inhibitors selective for human monoamine oxidase B ( MAO B) that do not inhibit MAO A have been described in the literature. The following compounds: 8-( 3- chlorostyryl) caffeine, 1,4- diphenyl- 2-butene, and trans, trans- farnesol are shown to inhibit competitively human, horse, rat, and mouse MAO B with K-i values in the low micromolar range but are without effect on either bovine or sheep MAO B or human MAO A. In contrast, the reversible competitive inhibitor isatin binds to all known MAO B and MAO A with similar affinities. Sequence alignments and the crystal structures of human MAO B in complex with 1,4- diphenyl- 2-butene or with trans, trans- farnesol provide molecular insights into these specificities. These inhibitors span the substrate and entrance cavities with the side chain of Ile- 199 rotated out of its normal conformation suggesting that Ile- 199 is gating the substrate cavity. Ile- 199 is conserved in all known MAO B sequences except bovine MAO B, which has Phe in this position ( the sequence of sheep MAO B is unknown). Phe is conserved in the analogous position in MAO A sequences. The human MAO B I199F mutant protein of MAO B binds to isatin ( K-i = 3 mu M) but not to the three inhibitors listed above. The crystal structure of this mutant demonstrates that the side chain of Phe- 199 interferes with the binding of those compounds. This suggests that the Ile-199 " gate" is a determinant for the specificity of these MAO B inhibitors and provides a molecular basis for the development of MAO B- specific reversible inhibitors without interference with MAO A function in neurotransmitter metabolism.