Stereochemistry and cofactor identity status of semicarbazide-sensitive amine oxidases

The relationship between the soluble copper topaquinone amine oxidases, the membrane bound semicarbazide-sensitive amine oxidases and lysyl oxidase remains unclear. The stereochemical course of substrate oxidation has been determined for each enzyme type and these studies suggest that SSAO and lysyl oxidase are closely related mechanistically, and that they are distinct from the copper amine oxidases. Both lysyl oxidase and SSAO catalyze the oxidation of tyramine with removal of the pro-S hydrogen from C-1 of this substrate. The copper amine oxidase enzymes that react with abstraction of the pro-S hydrogen from C-1 of substrates do not exhibit a solvent exchange pathway. In contrast, this exchange occurs in lysyl oxidase and SSAO reactions. The organic cofactor in all three enzyme types is a quinone; however, the spectral features of phenylhydrazine and p-nitrophenylhydrazine-derivatized SSAO differ from those reported for all known topaquinone-containing enzymes. Cofactor identification is further complicated by the lack of the characteristic topa motif, Asn-Tyr-Asp/Glu, in lysyl oxidase and the absence of any sequence information for SSAO.