Quinonoid dihydropteridine reductase, a tetrahydrobiopterin-recycling enzyme, contributes to 5-hydroxytryptamine-associated platelet aggregation in mice.

Quinonoid dihydropteridine reductase (QDPR) regenerates tetrahydrobiopterin (BH4), which is an essential cofactor for catecholamine and serotonin (5-hydroxytryptamine, 5-HT) biosynthesis. Serotonin is known as an important platelet agonist, but its role under BH4-synthesizing or recycling enzymes deficiency is unknown. In the present study, we evaluated the effect of Qdpr gene disruption on platelet aggregation using knockout (Qdpr-/-) mice. Platelet aggregation was monitored by light transmission aggregometry using adenosine diphosphate (ADP) and collagen as agonists. We also assessed how platelet aggregation was modified by 5-HT recovery through supplementation with 5-hydroxytryptophan (5-HTP), a 5-HT precursor, or by blocking the serotonin 5-HT2A receptor. Platelet aggregation in the Qdpr-/- mice was significantly suppressed in comparison with that in wild-type (Qdpr+/+) mice, particularly at the maintenance phase of aggregation. 5-HT storage was decreased in Qdpr-/- platelets, and 5-HTP supplementation recovered not only the intraplatelet 5-HT levels but also platelet aggregation. In addition, 5-HT signal blockade using sarpogrelate suppressed platelet aggregation in Qdpr+/+ mice, and platelets in Qdpr-/- mice were hardly affected. Our results indicate that QDPR deficiency suppresses platelet aggregation by impairing 5-HT biosynthesis in mice.

The role of thiols in the apparent activation of rat brain nitric oxide synthase (NOS).

The role of thiols on the activation and/or stabilization of rat brain nitric oxide synthase (NOS) has been investigated. It was found that thiols are not necessary for stabilizing or protecting the protein during purification but are required during enzyme turnover for maximum activity. In the complete absence of thiols but with added tetrahydrobiopterin, the enzyme retained a low basal activity. Thiol addition to a thiol-deplete preparation of the enzyme resulted in a 4 to 7-fold increase in activity when measured after 15 min. High concentrations of dihydropteridine reductase also caused an apparent activation of NOS and was capable of replacing thiols. The data presented is consistent with a cofactor role for thiols. The possibility that they serve as reducing agents for the regeneration of tetrahydrobiopterin from dihydrobiopterins is discussed.