Under anaerobic conditions, NADH produced in glycolysis builds up. This results in a reduction in the amount of NAD⁺ available to support continuation of glycolysis. Organisms have two pathways for regenerating NAD⁺ under anaerobic conditions. Animal cells and lactic acid bacteria use the process of lactic acid fermentation. Yeast convert pyruvate to acetaldehyde in a reaction catalyzed by the enzyme pyruvate decarboxylase. This is followed by reduction of acetaldehyde to ethanol catalyzed by alcohol dehydrogenase. The reaction uses NADH and releases NAD⁺, which is subsequently used in glycolysis.