COMT and Parkinson disease: Since Hcy is increased as a consequence of levodopa methylation by the enzymatic action of extracellular catechol-O-methyl transferase (COMT), use of a COMT inhibitor such as entacapone, the second most common medication administered for treatment of PD, may have beneficial effects for not only dopamine replacement therapy, but also protecting bone in levodopa-treated patients.