Abstract —A specific and sensitive, radioisotopic microassay for tryptophan hydroxylase (EC 1.14.36) is described, which is capable of determining enzymatic activity in as little as 5 μg of crude brainstem homogenate. 5‐Hydroxytryptophan, the immediate product of hydroxylation of tryptophan is enzymatically converted to N‐acetylserotonin. A radioisotopic label is then introduced by the enzymatic methylation of N‐acetylserotonin in the presence of [3H]methyl‐S‐adenosyl‐methionine. The [3H]‐melatonin thus formed is isolated by extraction and counted. With this assay, the activity in individual hypothalamic nuclei (arcuate nucleus, median eminence, suprachiasmatic nucleus, and medial forebrain bundle) has been measured.
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Kizer et al. (1975) studied this question.
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