64.對血清素（serotonin）的敘述，下列何者正確？
(A)大腦和腸道分泌大多為不同之亞型（subtype）
(B)消化道分泌量約占全身產量的90%
(C)由胱胺酸（cystine）所合成
(D)主要由胃的 parietal cells 所分泌

 Serotonin  (5-hydroxytryptamine;  5-HT)  is  present  in  highest concentration in blood platelets and in the gastrointestinal tract, where it is found in the enterochromaffi n cells and the myenteric plexus. It is also found within the brain stem in the midline raphé nuclei, which project to a wide portion of the CNS including the hypothalamus, limbic system, neocortex, cerebellum, and spinal cord 

Wikipedia 

Approximately 90% of the human body's total serotonin is located in the enterochromaffin cells in the GI tract, where it regulates intestinal movements

Biochemically, the indoleamine molecule derives from the amino acid tryptophan