Fig. 2.

Futile Cycles That Connect Multiple Subcellular Compartments and Tissues. (a) Glycerolipid-free fatty acid cycle between white adipose tissue and the liver. Triglycerides (TGs) are broken down to glycerol and free fatty acids (FFAs), which are either re-esterified in the adipose tissue or released into the bloodstream. The liver converts the glycerol from the bloodstream to glucose through gluconeogenesis, and this glucose can then be used to make glycerol-3-phosphate (G-3-P) needed for triglyceride synthesis. Free fatty acids picked up by the liver are used along with glycerol-3-phosphate for de novo triglyceride synthesis also called de novo lipogenesis. (b) Cori cycle between skeletal muscle and the liver. Glucose in the muscle is metabolized to pyruvate and ATP through glycolysis and then converted to lactate by lactate dehydrogenase (LDH), which is released into the blood and picked up by the liver. The liver uses the lactate to produce glucose, utilizing ATP, and the glucose is then released back into the circulation. Overall this futile cycle results in a net loss of 4 ATPs