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Inhibition of dopamine autoxidation by tetrahydrobiopterin and NADH in the presence of dihydropteridine reductase.

Dihydropteridine reductase (DHPR) catalyzes the regeneration of tetrahydrobiopterin (BH4) from quinonoid dihydrobiopterin by using NADH as a hydrogen donor. This enzymatic reaction has been found to serve as an antioxidation system during dopamine autoxidation in oxygenated Dulbecco's phosphate buffered saline (pH 7.5) at 37 degrees C. BH4 or NADH by itself has little or no inhibitory effect on dopamine autoxidation. The presence of DHPR, in addition to BH4 and NADH, greatly prolongs the lag period, which increases with increasing concentrations of each of BH4, NADH and DHPR. This BH4/DHPR-mediated antioxidation system is as effective as other antioxidation agents such as ascorbic acid, cysteine and reduced glutathione. Since BH4, NADH and DHPR are ubiquitous in mammalian tissues, this system may play an important role in the reduction of the oxidation products of catecholamines.