Deamination
While ammonia, derived mainly from the a-amino nitrogen of amino acids, is highly toxic, tissues convert ammonia to the amide nitrogen of nontoxic glutamine. Subsequent deamination of glutamine in the liver releases ammonia, which is then converted to nontoxic urea. The deamination of amino acids leaves a-keto acid carbon skeletons. Several of these a-keto acids are citric acid cycle intermediates. Amino acids are used to synthesize liver and plasma proteins, or their carbon skeletons are converted to glucose and glycogen by gluconeogenesis; the ammonia formed by deamination is converted to urea. The a-amino acids collected in the liver in the form of the amino group of L-glutamate molecules must be removed from glutamate to prepare them for excretion. In hepatocytes, glutamate is transported from the cytosol into mitochondria, where it undergoes oxidative deamination catalyzed by L-glutamate dehydrogenase. In mammals, this enzyme is present in the mitochondrial matrix.
