Aims/hypothesis. Reduced bioavailability of endothelium-derived nitric oxide is implicated in diabetic macrovascular and microvascular disease. In patients with diabetes, we hypothesised that protein glycosylation can alter nitric oxide binding affinity of haemoglobin and plasma proteins, hence reducing nitric oxide availability and causing an alteration in nitric oxide metabolism.

Methods. Binding of nitric oxide to haemoglobin was studied across a range of glycosylation levels in vitro (HbA_1c 5.9 to 9.8%). In clinical studies nitrate, nitrite, nitrosyl haemoglobin and plasma nitrosothiols were measured in venous blood from 23 patients with uncomplicated Type I (insulin-dependent) diabetes mellitus and 17 non-diabetic control subjects. Samples were analysed at baseline and after nitric oxide was added ex vivo.

Results. Nitric oxide-haemoglobin binding was increased at a HbA_1c greater than 8.5% compared with 5.9% (p<0.01). Basal nitrosyl haemoglobin was higher in diabetic patients compared with the control subjects (0.59±0.12 µmol/l vs 0.24±0.12 µmol/l, p<0.05). Plasma nitrosothiols, and nitrite and nitrate (NOx) concentrations were similar in diabetic patients compared with the control subjects (7.64±0.79 µmol/l vs 5.93±0.75 µmol/l, 13.98±2.44 µmol/l vs 12.44±2.15 µmol/l, respectively). In blood from diabetic patients, added nitric oxide was metabolised preferentially to nitrosyl haemoglobin and plasma nitrosothiols, with a twofold increase in nitrosyl haemoglobin observed across all concentrations of nitric oxide (p<0.05). These preferential increases correlated positively with HbA_1c.

Conclusion/interpretation. Nitrosyl haemoglobin is increased in patients with Type I diabetes. Preferential metabolism to nitrosyl haemoglobin and nitrosothiols occurs after increases in nitric oxide. Our results show an accentuated association between nitric oxide and glycosylated proteins, especially deoxygenated haem. An altered metabolic fate of nitric oxide could influence microvascular regulation and tissue perfusion.