Question

1. You feed pigs pyruvate that is isotopically labeled with 14C only at its methyl group (all other positions use standard natural abundance of 12C). Assuming that all of the pyruvate enters the TCA cycle, how many rounds of the TCA cycle are required before 14C is first released? Describe how you came to this conclusion..
2. Mammals lack the ability to biosynthesize glucose from acetyl-CoA, if a BALB/c mouse is given food containing 14C-labeled acetate, after some period of time it is possible to measure 14C in glycogen that is purified from the muscle tissue of the mouse. Knowing what you know about metabolism, please explain how this occurs.

Answer 1

a) During the fourth rounds of TCA cycle, 14C will be first released.

b) Acetate enters the TCA cycle by combining with oxaloacetic acid and forms citric acid. Citric acid goes through the TCA cycle till Oxaloacetate. Oxaloacetate can generate Phosphoenol pyruvate which generates glucose through the gluconeogenesis. Glycogen can be generated from glucose now. So, if you feed a mouse 14C-labeled acetate, it will generate glycogen, where you will be able to measure 14C.