Which enzyme reduces oxaloacetate to malate using NADH in the pathway?

• A. Pyruvate dehydrogenase
• B. Malate dehydrogenase
• C. Isocitrate dehydrogenase
• D. Malic enzyme

Answer: (B)

Malate dehydrogenase handles the interconversion between oxaloacetate and malate using NAD+/NADH as the redox partner. In the reductive direction, NADH donates a hydride to oxaloacetate to form malate, a reversible step that can occur in cellular contexts such as malate shuttles. The other enzymes perform different reactions: pyruvate dehydrogenase converts pyruvate to acetyl-CoA with CO2 release; isocitrate dehydrogenase oxidizes isocitrate to α-ketoglutarate; malic enzyme typically converts malate to pyruvate with NADP+, not reducing oxaloacetate to malate with NADH. Thus, the enzyme that matches the described reaction is malate dehydrogenase.