The urea cycle is a metabolic pathway that results in the formation of urea using one ammonium molecule from degraded amino acids, another ammonium group from aspartate and one bicarbonate molecule. This route commonly occurs in hepatocytes. The reactions related to the urea cycle produce NADH, and NADH can be produced in two different ways. One of these uses oxaloacetate. In the cytosol there are fumarate molecules. Fumarate can be transformed into malate by the actions of the enzyme fumarase. Malate is acted on by malate dehydrogenase to become oxaloacetate, producing a molecule of NADH. After that, oxaloacetate will be recycled to aspartate, as transaminases prefer these keto acids over the others. This recycling maintains the flow of nitrogen into the cell.