| Reaction | Pyruvate → lactate (reversible) | Pyruvate → acetyl-CoA (irreversible) | Pyruvate → oxaloacetate | Pyruvate → alanine (reversible) |
| Enzyme: lactate dehydrogenase (LDH) | Enzyme: pyruvate dehydrogenase complex | Enzyme: pyruvate carboxylase | Enzyme: alanine aminotransferase (ALT) |
| Present in heart, RBCs, muscle | Produces CO2, NADH+ + H+ | Requires biotin (vitamin B7), CO2, and ATP | Requires pyridoxal phosphate (active form of vitamin B6) |
| Occurs in anaerobic glycolysis | | | Occurs in states of muscle breakdown (catabolism) |
| Location | Cytosol | Mitochondrion | Mitochondrion | Cytosol of myocytes |
| Function | Lactic acid cycle (Cori cycle), lactate is released during anaerobic glycolysis | Acetyl-CoA enters the TCA cycle, ketogenesis, fatty acid synthesis | Intermediate for TCA cycle and gluconeogenesis | Alanine transports amino groups (from protein degradation) and carbons to the liver for gluconeogenesis (Cahill cycle) |
| Lactate is used for gluconeogenesis in heart and kidney | | | Liver converts alanine back into pyruvate, urea is a byproduct |
| NAD+ is replenished in anaerobic states by lactate formation | | | |
| Stimulated by | ↑ NADH/NAD+ ratio, anaerobic states (e.g., exercise), breakdown of ethanol | ↑ ADP, ↓ NADH/NAD+ ratio, ↑ Ca2+ | Acetyl-CoA | High protein intake, fasting, cortisol, epinephrine, and glucagon |
| Inhibited by | High lactate concentrations (feedback inhibition) | ↑ Acetyl-CoA/CoA ratio, ↑ NADH/NAD+ ratio, ↑ ATP/ADP ratio | | |
