Abstract
An unfortunate consequence of aerobic life is the structural damage to organic compounds such as DNA, proteins, carbohydrates and lipids that occurs as an effect of oxidative reactions. Oxidative damage inflicted by reactive oxygen species is termed “oxidative stress”. Biological systems contain powerful enzymatic and non-enzymatic antioxidant systems, and oxidative stress denotes a shift in the pro-oxidant/antioxidant balance in favour of the former. Diverse biological processes such as inflammation, carcinogenesis and ageing all appear to involve reactive oxygen species. There is also considerable evidence that oxidative stress resulting from increased production of reactive oxygen species (or their inadequate removal) plays a key role in the pathogenesis of diabetic complications.Dietary antioxidants have been suggested to decrease the risk of many chronic diseases. Curcumin is a natural antioxidant derived from turmeric (Curcuma longa) and has been recognised to possess therapeutic properties since ancient times. Much of the existing data for curcumin stem from experiments performed at supra-physiological concentrations (μM-mM) that are impossible to attain through oral ingestion. A consistent finding is that curcumin provides direct protection against ROS-mediated damage.
Glutathione is a crucial endogenous intracellular antioxidant that is tightly controlled by enzymes involved in its synthesis, utilization and breakdown. There is strong evidence of changes in glutathione peroxidase or total GSH levels, but little is known about changes in the rate-limiting enzyme for glutathione synthesis (glutamate-cysteine ligase; GCL) in pancreatic β–islet cells in diabetes. It was therefore hypothesized that curcumin at low concentration (those attainable in plasma after oral ingestion), though itself not acting as an antioxidant might trigger to cause a shift in the GSH/GSSG ratio in favour of former by upregulating the recycling and thus help combat oxidative stress and protect pancreatic β-islet cells.
The results indicated that curcumin, DMC and BDMC were able to scavenge hydroxyl radicals generated by menadione, but showed little scavenging ability against superoxide radicals. None of the curcuminoids were able to scavenge the NO generated using DETA NONOate. Plasma achievable nanomolar concentrations of curcuminoids are easily capable of preventing the deleterious effects of oxidative stress in pancreatic -islet RINm5F cells, at least in H2O2 model. The results point to an indirect method of protection, and one that favours protection against mitochondrial dysfunction over loss of membrane integrity. None of the curcuminoids showed a detrimental effect on insulin secretion, but the model did not allow assessment of any potential positive effect on insulin secretion. Further the findings confirmed that plasma attainable nanomolar concentrations of curcumin offered protection in pancreatic -islet RINm5F cells against H2O2-induced damage by modulating the proportion of GSSG:GSH in the favour of GSH and increasing the activity of SOD. This increase in GSH and SOD levels was, at least in part, on account of an increase in GR and SOD-2 gene expression. The intracellular mechanism driving this modulation of antioxidant gene was, by virtue of blocking the H2O2-induced NF-κB activation.
| Date of Award | 6 Jul 2011 |
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| Original language | English |
| Awarding Institution |
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| Sponsors | UHI Studentship |
| Supervisor | Ian Megson (Supervisor), Sandra MacRury (Supervisor), Irfan Rahman (Supervisor) & Garry Duthie (Supervisor) |