according to the 'anxiolytic paper' (2004), the enzyme would technically be INMT. not sufficiently expressed in the brain
The Index page for the reference article: Jacob MS, Presti DE Endogenous psychoactive tryptamines reconsidered: an anxiolytic role for dimethyltryptamine Med Hypotheses 2005 64(5):930-7
www.erowid.org
"A contemporary investigation, utilizing modern
genetic and structural techniques, has provided a
more detailed analysis of INMT, but does not provide
a complete story. In two studies, Thompson
et al. [35,36], cloned, expressed, localized, and
characterized the activities of rabbit and human
INMT. Using Northern blot analysis, they found rabbit
INMT transcripts expressed heavily in the lung,
moderately in the liver, and weakly in the brain. Human
INMT was expressed in the lung, thyroid, adrenal
gland, heart, muscle, and spinal cord, but not in
the brain. The authors observe high Km values (an
order of magnitude higher than in previous studies
[33,34]) of TYP for recombinant human INMT and
an absence of INMT mRNA transcripts in the brain.
Thus, Thompson et al. conclude that the production
of DMT in humans is not physiologically significant.
Their conclusion places much weight on the significance
of observed Km values for recombinant human
INMT and does not take into account several
additional genetic and enzymatic concerns."
but as you mentioned, current techniques have yet to quantitatively detect DMT formation
in situ.
as you've already figured as well, the typical concentration of DMT in serum is much lower than psychedelic doses
"In a short half-page report
in Nature, Franzen and Gross [8] reported the presence
of N,N-dimethyltryptamine in human blood
(8 · 10^-9 g/mL) and urine (4 · 10^-5 g/24 h).
Subsequent research found these levels to be too
high and that the average concentrations in normal
subjects tended to be around 5 · 10^-10 g/mL in
blood [12] and 4 · 10^-7 g/24 h in urine [23]. (It
should be noted that the threshold dose to produce
subjective effects in humans is about 5 · 10^-5
g/kg, which leads to peak blood concentrations
of 1 · 10^-8 g/mL [18,24])."
so the 'stress theory' of DMT production makes more sense, since INMT has been shown to be expressed in lungs, heart, adrenals, muscle and spinal tissues, the latter having high concentrations of serotonin. (the theoretical metabolic pathway for DMT synthesis would also inhibit serotonin production)
