Labelling of lipids by D-[1-14C] glucose, D-[6-14C] glucose and D-[3-3H] glucose in pancreatic islets from normal and GK rats

In pancreatic islets prepared from either normal or GK rats and incubated at either low (2.8 mM) or high (16.7 mM) D-glucose concentration, the labelling of both lipids and their glycerol moiety is higher in the presence of D-[1-C-14] glucose than D-[6-C-14] glucose. The rise in D-glucose concentration augments the labelling of lipids, the paired C-14/H-3 ratio found in islets exposed to both D-[1-C-14] glucose or D-[6-C-14] glucose and D-[3-H-3] glucose being even slightly higher at 16.7 mM D-glucose than that found, under otherwise identical conditions, at 2.8 mM D- glucose. Such a paired ratio exceeds unity in islets exposed to D-[1-C-14] glucose. The labelling of islet lipids by D-[6-C-14] glucose is about 30 times lower than the generation of acidic metabolites from the same tracer. These findings indicate (i) that the labelling of islet lipids accounts for only a minor fraction of D- glucose catabolism in pancreatic islets, (ii) a greater escape to L-glycerol-3-phosphate of glycerone-3-phosphate generated from the C-1-C-2-C-3 moiety of D-glucose than D-glyceraldehyde-3-phosphate produced from the C-4-C-5-C-6 moiety of the hexose, (iii) that only a limited amount of [3-H-3] glycerone 3-phosphate generated from D-[3-H-3] glucose is detritiated at the triose phosphate isomerase level before being converted to L-glycerol-3-phosphate, and (iv) that a rise in D- glucose concentration results in an increased labelling of islet lipids, this phenomenon being somewhat more pronounced in the case of D-[1-C-14] glucose or D-[6-C-14] glucose rather than D-[3-H-3] glucose.