malate dehydrogenase

**Reaction:** L-malate + NAD+ ⇌ oxaloacetate + NADH + H+ (reversible; the equilibrium strongly favors malate in standard conditions, but the reaction is pulled forward in the TCA cycle because oxaloacetate is rapidly consumed by citrate synthase).

**Isoforms:** - MDH1 (cytosolic): part of the malate-aspartate shuttle; also participates in gluconeogenesis (oxaloacetate is reduced to malate in mitochondria, exported, then re-oxidized to oxaloacetate in cytosol, generating cytosolic NADH needed for gluconeogenesis). - MDH2 (mitochondrial matrix): final step of the TCA cycle; regenerates oxaloacetate to condense with acetyl-CoA.

**Malate-aspartate shuttle:** Cytosolic NADH reduces oxaloacetate to malate (MDH1) → malate enters the matrix via the malate-α-ketoglutarate antiporter → MDH2 oxidizes malate to oxaloacetate, producing matrix NADH (feeds Complex I, ~2.5 ATP) → oxaloacetate is transaminated to aspartate (AST), which exits via the glutamate-aspartate antiporter. It is the main shuttle in liver, heart, and kidney, and yields more ATP than the glycerol-3-phosphate shuttle (which gives FADH2, ~1.5 ATP; predominant in skeletal muscle and brain in some descriptions). The malate-aspartate shuttle is reversible, unlike the glycerol-3-phosphate shuttle.

**Other roles:** - Gluconeogenesis: oxaloacetate (from pyruvate carboxylase in mitochondria) must be converted to malate to leave the mitochondria, then back to oxaloacetate for PEPCK. - Related to malic enzyme (malate → pyruvate, generating NADPH) in lipogenesis; this is a different enzyme and a common distractor.

**Regulation:** Product (NADH) inhibition; high NADH/NAD+ ratio shifts the reaction toward malate. In states with high NADH (e.g., ethanol metabolism), oxaloacetate is diverted to malate, impairing gluconeogenesis and the TCA cycle, which contributes to fasting hypoglycemia in alcoholics.

**Clinical significance/high-yield associations:** - Ethanol metabolism: alcohol dehydrogenase and acetaldehyde dehydrogenase raise NADH/NAD+ → pyruvate → lactate (lactic acidosis), oxaloacetate → malate (decreased gluconeogenesis → hypoglycemia), and promotes fatty acid synthesis/hepatic steatosis. - Hypoxia/ischemia: high NADH inhibits the TCA cycle, including MDH. - MDH2 deficiency: rare autosomal recessive cause of early-onset epileptic encephalopathy. MDH1 variants have also been reported in neurodevelopmental disease. - MDH1/MDH2 mutations: MDH2 is a tumor-susceptibility gene linked to hereditary paraganglioma/pheochromocytoma (along with SDHx, FH), supporting the TCA cycle–tumor link. - Arsenite and other inhibitors of TCA cycle enzymes (e.g., lipoic acid–dependent enzymes) do not directly inhibit MDH, which is not lipoate-dependent.

**TCA cycle mnemonic:** "Can I Keep Selling Sex For Money, Officer?" (Citrate, Isocitrate, α-Ketoglutarate, Succinyl-CoA, Succinate, Fumarate, Malate, Oxaloacetate). Malate → oxaloacetate generates the third NADH of the cycle (NADH from isocitrate DH, α-KG DH, and malate DH; FADH2 from succinate DH; GTP from succinyl-CoA synthetase). Per acetyl-CoA: 3 NADH, 1 FADH2, 1 GTP/ATP.

**Test pearls:** Know that MDH is NAD+-dependent (not FAD), the last TCA step, the cytosolic vs mitochondrial roles, and the effect of ethanol-induced high NADH/NAD+ ratio. Do not confuse with malic enzyme (NADP+-dependent, malate → pyruvate + CO2 + NADPH) or lactate dehydrogenase.