Nitrogen disposal also occurs during fasting when the carbon skeletons are used for gluconeogenesis or ketogenesis and in a number of situations that provoke negative nitrogen balance, such as serious trauma or essential amino acid deficiency. Nitrogens from proteins and other sources enter the urea cycle in the form of ammonia and aspartate. Although ammonia is generated in many tissues, it is transported to the liver in a safer form, namely glutamine. Glutamine is synthesized from glutamate and ammonia by glutamine synthetase. Within the liver, transaminases move amino groups from many different amino acids to alpha ketoacid acceptors forming aspartate and glutamate.
Ammonia also arrives at the liver from the intestine where it is generated in part by bacteria.
Thus the specific type of damage seen in fatty liver of pregnancy is likely to impair urea synthesis and lead to build up of ammonia. Failure to eat increases the load of nitrogen that the liver need to dispose of because fasting increases the breakdown of tissue protein. It is normally transported by serum albumin from its sites of production to the liver where it is further metabolized. Within the liver, bilirubin is made more polar and more readily excreted through a conjugation reaction that attaches 1 or 2 glucuronic acid groups. Conjugated bilirubin is normally excreted via the bile.