The major metabolite of dopamine, 3-methoxytyramine (3-MT), is monitored in equine urine samples from levodopa administration as an indicator of dopaminergic manipulation. Tyramine is a biogenic amine produced by decarboxylation of the amino acid L-Tyrosine. Changes in diet can alter the concentration of tyramine, making it a potential biomarker for monitoring dopamine abuse. The goal of this study was to measure and compare the 3-MT concentration after levodopa administration against a reference population. In addition, 3-mT/t values were examined to examine the suitability of tyramine as an endogenous reference compound (ERC).
UCT’s XTRACT column was used to extract urine samples. The samples were enzymatically hydrolyzed before being transferred to a SPE column. The matrix interference was washed off with a series of washing solutions. Target compounds were removed with ammonia in ethyl acetate. Extracts were evaporated and derivatized for analysis on GCMS.
The proposed reference limits for 3-MT and 3-MT/t were 776 ng/ml and 5.3, respectively. Of the two horses used in the levodopa administration study, horse 1 showed a maximum 3-MT concentration of 2866 ng/mL after 4 hours, which was identified as abnormal given the reference ranges. After 4 h the level of 3-MT in Horse 2 was 6314 ng/mL which would have been identified as positive for dopaminergic manipulation as it exceeded the 4000 ng/mL threshold.
Citation: Alborn M, Kavle A, Stanley S, Bowen C, Fu S. Intelligence gain of 3-methoxytyramine to tyramine ratio in equine urine. Drug test anal. 2022 Mar 28. DOI: 10.1002/dta.3264. Epub ahead of print. PMID: 35343638.