Investigation of relationship and protein-bound homocysteine between reduced, oxidized, and vascular endothelial function in healthy human subjects

Previous studies investigating homocysteine and vascular disease have relie d on total plasma homocysteine as the sole index of homocysteine status. We examined the dynamic relationship between vascular endothelial function an d concentrations of total, protein-bound oxidized, free oxidized, and reduc ed homocysteine to identify the homocysteine form associated with endotheli al dysfunction in humans. We investigated 14 healthy volunteers (10 men, 4 women). Brachial artery flow-mediated dilatation was measured at baseline a nd at 30, 60, 120, 240, and 360 minutes after oral (1) L-methionine (50 mg/ kg), (2) L-homocysteine (5 mg/kg), and (3) placebo. Plasma concentrations o f total, protein-bound oxidized, free oxidized, and reduced homocysteine we re measured at each time point, and nitroglycerin-induced dilatation at was assessed at 0, 120, and 360 minutes. Flow-mediated dilatation fell, and co ncentrations of total, protein-bound oxidized, free oxidized, and reduced h omocysteine increased after oral homocysteine and oral methionine (all P<0. 05 for difference in time course compared with placebo). Flow-mediated dila tation showed a reciprocal relationship with reduced homocysteine during bo th homocysteine and methionine loading. In both loading studies, peak reduc tion in flow-mediated dilatation coincided with maximal reduced homocystein e concentrations. In contrast, there was no consistent relationship between flow-mediated dilatation and free oxidized homocysteine, protein-bound oxi dized homocysteine, or related species. Nitroglycerin-induced dilatation wa s unchanged by oral homocysteine and oral methionine (P>0.10 compared with placebo). Reduced homocysteine is closely associated with endothelial dysfu nction during oral methionine and oral homocysteine loading. Our observatio ns support the hypothesis that reduced homocysteine is the deleterious form of homocysteine for vascular function in vivo and suggest a less important role for other homocysteine species.