IN VITRO STUDIES ON THE INTERACTION OF BRAIN MONOAMINE OXIDASE WITH 5,6‐ AND 5,7‐DIHYDROXYTRYPTAMINE12

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Abstract
[14C]5,6‐Dihydroxytryptamine ([14C] 5,6‐DHT) and [14C]5,7‐dihydroxytryptamine ([14C]5,7‐DHT) were deaminated to toluene‐isoamylalcohol extractable products when incubated with homogenates of rat hypothalamus or pons‐medulla oblongata. [14C]5,6‐Dihydroxyindole acetic acid ([14C]5.6‐DHIAA) and [14C]5,7‐dihydroxyindole acetic acid ([14C]5,7‐DHIAA) were detected as MAO metabolites by TLC besides non‐identified components. The conversion of [14C]5,6‐DHT and [14C]5,7‐DHT obeyed, at least initially, Michaelis‐Menten kinetics (Km 5,7‐DHT: 0.5 × 10−3M; Km 5,6‐DHT: 1.25 × 10−3M). Inhibition of the reaction by the MAO A inhibitor, clorgyline, resulted in a typical double sigmoidal inhibition curve indicating that both amines are metabolized by both types of MAO (A and B). In deprenyl inhibition studies, however, 5,7‐ and 5,6‐DHT seemed to be preferred substrates of MAO A.Incubation of rat brain homogenates with [14C]5,6‐DHT and [14C]5,7‐DHT or with the MAO metabolites [14C]5,6‐DHIAA and [14C]5,7‐DHIAA caused a time‐dependent break‐down of the dihydroxylated indole compounds with subsequent binding of radioactivity to perchloric acid insoluble tissue components.5,6‐DHT inactivated MAO in rat brain homogenates parallel to its decomposition and extensive protein binding. The inactivation of MAO by 5,6‐DHT and the extensive binding of radioactivity to protein were antagonized by dithiothreitol (DTT), glutathione (GSH) and L‐ascorbic acid. Reduction of [O2] in the incubation medium slightly attenuated the inactivation of MAO by 5,6‐DHT. Catalase or superoxide dismutase failed to prevent MAO from being inactivated by 5,6‐DHT. The results suggest that oxidation products of 5,6‐DHT, e.g. its corresponding o‐quinone, are involved in the inactivation of MAO in vitro and mainly responsible for the binding of radioactivity to brain proteins in vitro. Similar mechanisms may also be operative in the in vivo neurotoxicity of 5,6‐DHT.The lack of inactivation of MAO by 5,7‐DHT in vitro correlated with a low degree of radioactivity binding (from [14C]5,7‐DHT) to homogenate protein pellets; the binding to proteins was barely influenced by GSH, cysteine, DTT and l‐ascorbic acid. These latter findings do not provide a plausible explanation for the mechanism(s) involved in the well known in vivo neurotoxicity of 5,7‐DHT.