There are 3 transcobalamins; TC1, TC2 and TC3. When they have a cobalamin bound in place they become HOLOTranscobalamin 1,2 and 3 respectively. TC1, hapiticorin, is in the saliva and other places. TC2, become HTC2, transports cobalamins to the cells. HTC3 transports misc cobalamins back to the liver for disposal.
Taking injections or sublingual b12 make the question of TC1, 2, and 3 all irrelevant as they are involved in the transport of b12 one place or another in a minimal, at best, fashion. The b12 is transported by diffusion to ALL the cells needing it when one absorbs about 100mcg a day of AdoCbl/MeCbl. The two active b12s when transported by diffusion are subject to being filtered out by the kidneys and subject to destruction by cyanide, nitrous oxide, glutathione and other substances, hence the short half life. Direct diffusion effectiveness of MeCbl and AdoCbl can be distinctly felt by many people starting in 5 to 10 minutes after a sublingual goes into ones mouth. TC2 only matters if one is depending upon it to transport IF absorbed b12 or inactive cobalamins taken apart and reassembled to be transported to the cells in limited quantity. The presence or lack there of of TC2 makes no differencve to directly absorbed AdoCbl and/or MeCbl.
Five ml of semen contains as much TC2/HTC2 as 1000ml of blood. That appears to indicate that 5ml of semen can potentially transport as much b12 as 1 liter of blood. That could be as much as an entire microgram or two. That must be evolutionarily important as that is a fairly expensive gift. As it is bound to TC2 it is ready for delivery and perhaps very readily absorbable through vaginal mucosa.
The so-called "active" b12 test is actually a test of TC2 and HTC2 based on the assumption that when that is saturated a person is maxed out on b12 and CAN'T make use of any other b12. It's very assumptions don't allow for distribution by diffusion and the enormous difference in healing that makes. It appears part of the strategy to limit b12 doses to about 25 mcg without a prescription.
