Effect of vitamin C on the availability of tetrahydrobiopterin in human endothelial cells

Vitamin C has long been known for its beneficial vascular effects, but its mechanism of action remains unclear. Recent reports suggest that vitamin C may prevent endothelial dysfunction by scavenging free radicals and increas ing the bioavailability of nitric oxide. To investigate this area further, we studied the effect of vitamin C (10(-4) M) and Mn(III) tetrakis (4-benzo ic acid) porphyrin chloride (MnTBAP; 10(-5) M), a scavenger of superoxide, hydrogen peroxide, and peroxynitrite, on endothelial nitric oxide synthase (eNOS) enzymatic activity in cultured human umbilical vein endothelial cell s, L-Citrulline formation (a measure of eNOS enzymatic activity) was signif icantly increased in cells treated for 24 h with vitamin C. No effect was o bserved after MnTBAP treatment. Chronic administration of vitamin C also ha d no effect on eNOS protein expression. Treatment with vitamin C for 24 h s ignificantly increased levels of the eNOS co-factor tetrahydrobiopterin (BH 4), whereas MnTBAP did not affect its levels. Sepiapterin (10(-4) M), a pre cursor of BH4, significantly increased eNOS activity, whereas addition of v itamin C to cells treated with sepiapterin did not cause any further increa se in eNOS activity. Our results suggest that the beneficial effect of vita min C on endothelial function is best explained by increased intracellular BH4 content and subsequent enhancement of eNOS activity. This effect appear s to be independent of the ability of vitamin C to scavenge superoxide anio ns.