To the Editor: We read with interest the recent paper by Looker et al. [1] who presented a study on homocysteine as a risk factor for nephropathy and retinopathy in Type 2 diabetes. We would like to add some comments and observations relating to possible underlying mechanisms that may be of interest.

Pathophysiologically, both diabetes mellitus and hyperhomocysteinaemia are associated with oxidant stress, in particular the formation of superoxide [2]. We found that in aortae of diabetic rabbits, homocysteine augments the impairment of acetylcholine-stimulated relaxation and cGMP formation, in vitro, but had no effect in age-matched control rabbits [2]. These effects were reversed with superoxide dismutase (SOD), indicating that homocysteine augments superoxide formation in aortae from diabetic animals. Since superoxide reacts with nitric oxide (NO) to form peroxynitrite (ONOO), effectively reducing the bioavailability of NO [2], this readily explains the observed effects in our studies. Since diminished NO formation is now firmly associated with vasculopathy [2], these data consolidate that homocysteine and diabetes mellitus interact to augment diabetic angiopathy, in part, through diminished NO formation.

These observations could also explain why increased concentrations of homocysteine are not always associated with diabetic angiopathy [3]. We showed that 10 µmol/l homocysteine (considered within the normal range in humans) is sufficient to inhibit NO formation in aortae from diabetic animals, whereas in aortae from non-diabetic animals, concentrations of homocysteine as high as 1 mmol/l are required to elicit similar effects [2]. Thus, plasma concentrations of homocysteine might not necessarily be at the “risk factor level” for homocysteine to have an angiopathic effect in diabetic patients.

We also established that the potency of homocysteine on the inhibition of NO-mediated relaxation was potentiated by copper in the normal rat aorta [4]. It was therefore suggested that it may not be the absolute concentrations of homocysteine alone but the relative concentrations of copper that determines the vaculopathic effect of homocysteine through an augmentation of the auto-oxidation of the amino acid [4]. It is of interest, therefore, that copper concentrations are increased in the plasma of patients with both hyperhomocysteinaemia and diabetes mellitus and that increased concentrations of copper are themselves a risk factor for cardiovascular disease [5]. It has also been reported that plasma copper concentrations are correlated with increased homocysteine concentrations in patients with peripheral arterial disease [6]. To consolidate this proposal, we found that copper augmented the impairment of acetylcholine-stimulated relaxation and cGMP formation in aortae from diabetic but not the control rabbits matched for age [7]. We are currently investigating homocysteine-copper interactions in the same animal model. All studies cited using animal models were given humane care in compliance with the National Institute of Health Guidelines for animal research and were approved by the United Kingdom Home Office.

Finally, since diabetes mellitus and hyperhomocysteinaemia are both associated with the overproduction of O2 – and a reduction of NO formation in arterial tissue [5], it is reasonable to speculate that diabetes mellitus and homocysteine and copper exert an interactive negative effect on NO formation through a common denominator system(s). There are several candidate mechanisms: (i) diabetes mellitus augments the intra-arterial (auto)oxidation of homocysteine and therefore the generation of superoxide; (ii) homocysteine and copper further impair the already reduced activity of SOD and other antioxidant systems; (iii) homocysteine and copper increase the activity of enzymes that generate superoxide [e.g. nicotinamide adenine dinucleotide phosphate (NADPH) oxidase].

Further studies on the interactions between diabetes mellitus, homocysteine and copper on these and other systems are required to determine whether they play a role in diabetic angiopathy. These lines of evidence also point to the potential use of antioxidants, in particular SOD mimetics, to treat diabetic angiopathy.

N. Shukla, G.D. Angelini, J.Y. Jeremy