As part of a research experiment, a person undergoes two fine needle muscle biopsies to obtain small amounts of tissue for biochemical analysis. The first is taken at the beginning of the experiment while the subject is at rest. The second is taken at the end of 30 minutes of aerobic exercise on a stationary bicycle. The activity of the muscle pyruvate dehydrogenase complexes is found to be much higher in the second measurement. Which of the following biochemical changes would be most likely to produce this effect?

A. Decreased ADP
B. Decreased intracellular Ca2+
C. Increased acetyl CoA
D. Increased NADH/NAD+
E. Increased pyruvate concentration

My guess is D. Increased NADH/NAD+

The correct answer is E. In most biochemical pathways, only one to a few enzymatic reactions are under regulatory control. These often occur either at the beginning of pathways or at pathway branch points. The pyruvate dehydrogenase complex controls the link between glycolysis and the citric acid cycle, and decarboxylates pyruvate (the end product of glycolysis) with production of NADH and acetyl CoA (the substrate for the citric acid cycle). Pyruvate dehydrogenase (PDH) in mitochondria is huge, visible as dots by electron microscopy, because it is composed of many copies of at least 5 separate enzymes. Three of these (pyruvate dehydrogenase, dihydrolipoyl transacetylase, and dihydrolipoate dehydrogenase) perform the actual biochemical reactions. The other two, PDH kinase and phosphatase, add or remove phosphate groups from the PDH complex, respectively. The more active form of PDH is not phosphorylated; the less active form is. In exercise, high ADP and pyruvate concentrations inhibit PDH kinase, while increased Ca2+ stimulates PDH phosphatase. Both effects produce more of the dephosphorylated, more active form of PDH.
A higher, not decreased, concentration of ADP (choice A) is seen with exercise and tends to turn PDH on.

Intracellular calcium (choice B) is increased, not decreased, with exercise and tends to turn PDH on.

Increased acetyl CoA (choice C), as seen in active lipolysis, tends to turn PDH off.

Increased NADH/NAD+(choice D), as is seen when the cell's needs for reduced NADH are being adequately met already, tends to turn PDH off.