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Metabolism and Enzymology

N,N-Dimethyltryptamine Production in Phalaris aquatica Seedlings

A Mathematical Model for its Synthesis

Chemistry Department, University of Maryland Baltimore County, Catonsville, Maryland 21228, Department of Biochemistry, University of Sydney, Sydney, N.S.W., 2006, Australia

The activities of three enzymes and the concentration of intermediates involved in the synthesis of N,N-dimethyltryptamine (DMT) from endogenous tryptophan (TRP) have been measured in vitro in seedlings of Phalaris aquatica L. cv Australian Commercial over 16 days after planting. The activities of tryptophan decarboxylase and the two N-methyl-transferases increased rapidly to maximal rates of substrate conversion at day 5 of 95, 1000, and 2200 micromoles per hour per milliliter, respectively. After these maximal rates, the activities decreased rapidly. The concentration of intermediates increased rapidly from zero in the seeds to maximal values of 25 and 53 micromolar at day 5 for tryptamine (T) and N-methyltryptamine (MT), respectively, 1000 micromolar at day 6 for TRP, and 650 micromolar at day 8 for DMT. The concentration of DMT and of all the intermediates in its synthesis declined rapidly after the maximal value had been reached. A mathematical model of the pathway from TRP to DMT using these enzymes correctly predicts the concentrations of T and MT, intermediates whose concentration is determined only by the pathway, and confirms that these three enzymes are responsible for the in vivo synthesis of DMT. Kinetic studies are reported for these enzymes. Tryptophan decarboxylase uses pyridoxal phosphate (PALP) as a coenzyme and has the following kinetic constants: K_m^PALP = 2.5 micromolar, K_m^TRP = 200 micromolar, K_i^MT = 5 millimolar, and K_i^DMT = 4 millimolar. The N-methyltransferases use S-adenosylmethionine (SAM) as substrate; S-adenosylhomocysteine (SAH) is assumed to be the product. The mechanism of secondary indolethylamine-N-methyltransferase, determined by initial velocity studies, is rapid equilibrium random with formation of both dead end complexes. Secondary indolethylamine-N-methyltransferase methylates both MT and 5-methoxy-N-methyltryptamine (5MeOMT). The kinetic constants for the methylation of MT are: K_MT = 40 ± 6, K_SAM = 55 ± 15, K_DMT = 60, K_SAH = 4.3 ± 0.4 micromolar with unity interaction factors. The kinetic constants for the conversion of 5MeOMT to 5-methoxy-N,N-dimethyltryptamine (5MeODMT) are K_5MeOMT = 40 ± 10, K_SAM = 90 ± 40, and K_SAH = 2.9 ± 0.3 micromolar with unity interaction factors, except for SAM-5MeODMT = 2.0 ± 0.9 and SAH-5MeOMT = 0.45 ± 0.25. The kinetic constants for primary indolethylamine N-methyltransferase are K_m^T = 20, K_m^SAM = 40, K_i^DMT = 450 micromolar with the substrates binding independently.