Which tissue primarily uses the glycerol-3-phosphate shuttle for transferring reducing equivalents into mitochondria?

• A. Skeletal muscle
• B. Liver
• C. Heart
• D. Brain

Answer: (A)

The glycerol-3-phosphate shuttle moves electrons from cytosolic NADH into the mitochondria by converting DHAP to glycerol-3-phosphate, then transferring those electrons to FAD on mitochondrial glycerol-3-phosphate dehydrogenase to form FADH2 that enters the electron transport chain at Complex II. This pathway bypasses Complex I and yields a bit less ATP per NADH than the malate-aspartate shuttle, but it can operate rapidly to regenerate NAD+ during intense glycolytic activity. Skeletal muscle relies on this shuttle to quickly sustain glycolysis during contraction, making it the tissue where glycerol-3-phosphate is the primary route for transferring reducing equivalents into mitochondria. Heart, liver, and brain more commonly use the malate-aspartate shuttle, which transfers electrons with higher ATP yield per NADH.