Fig. 1. Substrate cycles in glucose metabolism. Sites of ³H loss: ³H at C-2 of the glucose molecule is lost in the interconversion of glucose-6-phosphate (G6P) and fructose-6-phosphate (F6P); ³H at C-6 is retained through the glucose and fructose cycles and lost in the PEP and citric acid cycles. PEP, phosphoenolpyruvate; F1,6-bP, fructose-1,6 bisphosphate.