Inhibition of brain mitochondrial monoamine oxidases by the endogenous compound 5-hydroxyoxindole.

5-Hydroxyoxindole is a recently identified endogenous compound. Its physiological role remains unclear but certain evidence exists, that it may share some regulatory properties with isatin, a known endogenous inhibitor of monoamine oxidase (MAO) type B (MAO-B). In this study several oxidized indoles were tested for their in vitro inhibition of MAO type A (MAO-A) and B of rat brain non-synaptic mitochondria. 5-Hydroxyoxindole was less potent MAO-A inhibitor (IC50 56.8 microM) than isatin (31.8 microM) and especially 5-hydroxyisatin (6.5 microM), but it was the only highly selective MAO-A inhibitor among the all compounds studied (IC50 MAO-A:IC50 MAO-B = 0.044). Thus, the in vitro data suggest that MAO-A may represent potential target for 5-hydroxyoxindole.

Detection and quantification of 5-hydroxyoxindole in mammalian sera and tissues by high performance liquid chromatography with multi-electrode electrochemical detection.

OBJECTIVE: Since 5-hydroxyoxindole structurally related indole metabolites play different roles in some hepatic and neurologic disorders we found necessary to develop an assay to further investigate the physiologic relevance of this compound. METHODS: We have designed a convenient assay to determine 5-hydroxyoxindole in serum using solid phase extraction and a highly selective High Performance Liquid Chromatography system with multi-Electro Chemical Detection (HPLC-ECD). RESULTS: We have identified and quantified 5-hydroxyoxindole in various mammalian species. Its distribution in tissues showed that the molecule is also present in brain, liver, kidney and spleen, but not in skeletal muscle. CONCLUSIONS: 5-hydroxyoxindole is an endogenous tryptophan metabolite present in circulating blood and in some tissues at the nmol level, its determination using HPLC-ECD will be useful for elucidating the role of this molecule in normal and disease conditions.