The major methyltransferases are: COMT (catechol-O-methyltransferase) : inactivates catecholamines (dopamine, norepinephrine, epinephrine), catechol estrogens, and catechol drugs (L-DOPA) R HNMT (histamine N-methyltransferase) : the primary intracellular histamine degradation enzyme in the brain and peripheral tissues R INMT (indolethylamine N-methyltransferase) : methylates tryptamine, serotonin, and other indoleamines R NNMT (nicotinamide N-methyltransferase) : methylates nicotinamide (vitamin B3) and other pyridines R TMT (thiol S-methyltransferase) : S-methylates thiol drugs (captopril, D-penicillamine) and thiol metabolites R TPMT (thiopurine S-methyltransferase) : methylates thiopurine drugs (azathioprine, 6-mercaptopurine) used in autoimmune disease and cancer R Methylation capacity depends on the methylation cycle: methionine to SAM-e to SAH to homocysteine, recycled back to methionine by methionine synthase (B12-dependent) or BHMT (betaine-dependent)
Diarrhea, conjunctivitis, alopecia, and photophobia are also observed (254, 256)
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B12 Intramuscular Dose Recommendations The usual dose for vitamin B12 injections is 1000 mcg (1 mg)
