I'm not sure. B12 doesn't need 5-THF to get remethylated, does it? That's not an equation I'm familiar with. My understanding is that MTRR methylates B12 so it can be used (in active form) by MTR, and you need methylfolate also for MTR to work. (But I think maybe I got corrected once - that MTRR isn't methylating but activating the methyl B12? Or maybe it was that the methyl group doesn't leave the B12 for every homocysteine that gets turned into methionine - only the methyl from the methylfolate gets transferred. )
Good point... B12 does or does not need to get remethylated? I thought I had it backwards so I went back to an 1hr long video about the MTR/MTRR cycle by Dr.Ben Lynch. This is not publicly available because it's part of a long 10hr+ lecture he gave at then end of 2013 and it's sold as a seminar on seekinghealth.org.
In any case in the video he's talking about Methyl-B12 donating the CH3 group to Homocysteine becoming cobalamin ("naked" as he calls it) and says:
"Methylfolate comes in and donates the methyl group, so now methylfolate isn't methylfolate anymore it is tetrahydrofolate (THF)"
...which is still not clear because he's not saying where the CH3 goes. But a few seconds later he says:
"Methlyfolate gives the methy group back to the cobalamin and turns into tetrahydrofolate to restart the folate cycle..."
So it seems to me that the B12 is getting remethylated.
Of course I suppose this doesn't go on indefinitely otherwise it would be "perpetual motion" and we wouldn't need any more methyl-B12, it just regenerates itself ...
I suppose at one point the cobalamin gets oxidized and won't work any longer.
Here's an article that goes a bit in detail of this process:
The purpose of the methionine synthase enzyme is to transfer methyl groups. This process begins when the B12-cobalt domain receives a methyl group from methyl-tetrahydrofolic acid domain and subsequently hands it off to homocysteine. When it is empty, the B12-cobalt site requires “protection” by the CAP domain so that excessive oxidation does not occur. When the cobalt atom does get oxidized, the SAM domain can repair it by donating its methyl group. Dr. Deth discovered that the superstructure of methionine synthase is different in the brain than it is in the body and that brain methionine synthase can be missing critical “domains” or functional sites (i.e. the SAM and CAP domains). When these sites are absent, the oxidized brain B12 is actually able to dissociate and come off the methionine synthase enzyme and be replaced by a methyl-B12. Because of this unique characteristic where the methionine synthase enzyme continually loses its oxidized B12 in the cortex of the brain, methyl-B12 must
continually be replenished in order for the methionine synthase/methyl-B12 complex to reach its maximum clinical potential for children on the autistic spectrum.At least in autism it is because of this phenomenon that we find ourselves dealing with a relative methyl-B12 dependency more than we find
ourselves dealing with an actual methyl-B12 deficiency.
http://www.californiahyperbarics.com/autism21.html
cheers