Radioimmunoassay of 5-methoxytryptophol in plasma

5-Methoxytryptophol (ML) is synthesized by the pineal gland, but no radioimmunoassay has been described for its routine measurement in human plasma. We have developed and validated such an RIA. The assay is sensitive (detecting as little as 8 ng/L) and specific, and requires no extraction stage. A preliminary study of healthy volunteers showed an intra-individual variation in plasma ML that was independent of the sex of the subject and the time of daytime collection. Investigation of the 24-h pattern of ML in seven men revealed a low-amplitude daily rhythm (P less than 0.03). Mean concentrations of ML between noon and midnight significantly exceeded those between 0030 and 1130 hours in each individual (P less than 0.05). This assay is practical and convenient, and it should greatly assist in investigation of factors affecting concentrations of ML in human plasma.