Catechol-O-methyltransferase decreases levodopa toxicity in vitro

The purpose of this study was to examine the effects of 3-O-methylation by catechol-O-methyltransferase (COMT) on the toxicity of levodopa in neuronal cultures. High concentrations of levodopa an toxic in vitro. Therefore, th ere is concern that longterm treatment with levodopa in patients with Parki nson's disease might accelerate the rate of degeneration of nigrostriatal n eurons. However, recent studies have suggested that, while levodopa is harm ful in vitro, it may not be toxic in vivo. A possible defense mechanism is by means of metabolic shunting of levodopa excess to 3-O-methyldopa by COMT in peripheral and central nervous system tissues. In this study we examine whether the use of COMT inhibitor, which reduced the levels of 3-O-methyld opa, affect levodopa toxicity. Mice cerebellar granule neurons, PC12, and n euroblastoma cells were used, and their viability following exposure to lev odopa and COMT with and without tolcapone, a COMT inhibitor, was measured b y neutral red staining. Auto-oxidation of levodopa was evaluated using a sp ectrophotometer (690 nm). We found that 3-O-methyldopa, unlike levodopa, wa s not toxic to all cells examined. Addition of purified COMT to levodopa pr evented its auto-oxidation and markedly attenuated its cytotoxicity in vitr o. Additional tolcapone reversed the protective effect of COMT. The agent 3 -O-methyldopa is not toxic to cell cultures. Catechol-O-methyltransferase a ttenuates toxicity of levodopa in vitro by its metabolism to nontoxic 3-O-m ethyldopa.