Tetrahydrobiopterin is released from and causes preferential death of catecholaminergic cells by oxidative stress

The underlying cause of the selective death of the nigral dopaminergic neur ons in Parkinson's disease is not fully understood. Tetrahydrobiopterin (BH 4) is synthesized exclusively in the monoaminergic, including dopaminergic, cells and serves as an endogenous and obligatory cofactor for syntheses of dopamine and nitric oxide. Because BH4 contributes to the syntheses of the se two potential oxidative stressors and also undergoes autoxidation, there by producing reactive oxygen species, it was possible that BH4 may play a r ole in the selective vulnerability of dopaminergic cells. BH4 given extrace llularly was cytotoxic to catecholamine cells CATH.a, SK-N-BE(2)C, and PC12 , but not to noncatecholamine cells RBL-2H3, CCL-64, UMR-106-01, or TGW-nu- 1. This was not caused by increased dopamine or nitric oxide, because inhib ition of their syntheses did not attenuate the damage and BH4 did not raise their cellular levels. Dihydrobiopterin and biopterin were not toxic, indi cating that the fully reduced form is responsible. The toxicity was caused by generation of reactive oxygen species, because catalase, superoxide dism utase, and peroxidase protected the cells from the BH4-induced demise. Furt hermore, thiol agents, such as reduced glutathione, dithiothreitol, beta-me rcaptoethanol, and N-acetylcysteine were highly protective. The BH4 toxicit y was initiated extracellularly, because elevation of intracellular BH4 by sepiapterin did not result in cell damage. BH4 was spontaneously released f rom the cells of its synthesis to a large extent, and the release was not f urther enhanced by calcium influx. This BH4-induced cytotoxicity may repres ent a mechanism by which selective degeneration of dopaminergic terminals a nd neurons occur.