The biosynthesis of 5-hydroxytryptamine in brain.

1. Studies on the anatomical distribution of tryptophan 5-hydroxylase in dog brain show that the activity of this enzyme parallels the activity of 5-hydroxytryptophan decarboxylase and the concentration of 5-hydroxytryptamine in various areas of the brain. 2. Subcellular fractionation of homogenates of rabbit hind-brain has shown that at least 40% of the tryptophan 5-hydroxylase activity is associated with the crude mitochondrial fraction; equilibrium centrifugation in a discontinuous sucrose gradient suggests that this particulate tryptophan 5-hydroxylase is localized in the synaptosomes (nerve-ending particles). 3. Attempts to increase the activity of tryptophan 5-hydroxylase in iso-osmotic homogenates of brain by adding 5,6,7,8-tetrahydro-6,7-dimethylpteridine failed. 4. Procedures designed to rupture synaptosomes caused a fall in the activity of tryptophan 5-hydroxylase, which fall was reversed by adding 5,6,7,8-tetrahydro-6,7-dimethylpteridine. This suggested either that the intact synaptosome, in which the tryptophan 5-hydroxylase is localized, contained an optimum amount of the pteridine, or that the pteridine could not pass across the nerve-cell membrane. 5. Partial purification of tryptophan 5-hydroxylase from brain preparations by ammonium sulphate precipitation revealed that the enzyme was completely dependent on 5,6,7,8-tetrahydro-6,7-dimethylpteridine for activity.