PRECURSR
Precursor in the biosynthesis of 5-HT is tryptophan it is converted to 5-hydroxytryptophan (in chromaffin cells and neurons but not in platelets) by the action of tryptophan hydroxylase (an enzyme confined to 5-HT-Producing cells) this is the rate limiting step the 5-hydroxytryptophan is then decarboxylated to 5-HT by the same amino acid decarboxylase that participates in the synthesis of catecholamines and histamine platelets (and neurons) possess a high affinity 5-HT uptake mechanism and platelets become loaded with 5-HT as they pass through the intestinal circulation where the local concentration is relatively high the mechanisms of synthesis storage release and reuptake of 5-HT are very similar to those of noradrenaline and many drugs affect both processes indiscriminately recent work has shown that 5-HT is often stored in neurons and chromaffin cells as a co-transmitter together with various peptide hormones such as somatostatin substance P or vasoactive intestinal polypeptide (VIP)
Degradation of 5-HT occurs mainly through oxidative deamination catalysed by monoamine oxidase followed by oxidation to 5-hydroxyindoleacetic acid (5-HIAA) the pathway being exactly analogous to that of noradrenaline catabolism 5-HIAA is excreted in the urine and serves as an indicator of 5-HT production in the body this is used for example in the diagnosis of carcinoid syndrome 
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