Albinism results from a lack of the enzyme tyrosine hydroxylase, in turn causing albinos to lack melanin, which pigments skin, hair, and eyes. In urea synthesis, ammonia and aspartate contribute nitrogen, while HCO3- provides carbon. The product and ornithine combine to form citrulline, which reacts with aspartate to acquire the second nitrogen of urea. Ultimately, arginine is formed, and its cleavage yields urea and regenerates ornithine. See pages 84-85 and page 93 of textbook for more information on the above.
Tyrosine is a precursor to a class of neurotransmitters called catecholamines.
Low levels of L-DOPA are associated with Parkinson's disease. Tryptophan is a precursor to serotonin, which has a sedative effect, giving a pleasant feeling. Milk proteins have high levels of tryptophan. Very low levels of seratonin are associated with depression, while extremely high levels actually produce a manic state. Manic-depressive schizophrenia can actually be controlled by monitoring the levels of serotonin.