Melatonin is derived from amino acid L-tryptophan. Is there any difference between ELISA specificity to the endogenous (naturally occurring) and the exogenous (drug or implants) melatonin?
As melatonin is a single, discrete substance, an ELISA should not be able to differentiate the source. However, the result could be affected by other substances in a mixture containing the melatonin if they affect its binding to the assay plate.
Thanks Dr Shapiro for your answer. In our experiment, we treated the animals with melatonin implants (commercially designed for the sheep), however, when we measure serum melatonin level, it was lower than the control animals. That is why I am surprised by this result.
A melatonin implant will result in 24h melatonin levels and by that interfere with the endogenous circadian melatonin rhythm. This can have a negative influence on several metabolic processes that also have a 24h circadian rhythm.