I have reread the Quadros
review of cobalamin assimilation and metabolism plus a
two reviews on the cobalamin letter diseases (inborn errors of metabolism) which help to define the steps in intracellular cobalamin processing.
I was reminded all over again of my original frustration in trying to understand the detail better - every interesting reference in the Quadros review is behind a significant pay wall.
I found out though that Quadros is very active in cobalamin research and has made significant contributions to understanding. So until evidence is produced to the contrary, I'll accept the statements made in the review even if I can't read the detail behind them.
The reviews of the letter diseases (described as cblA - cblG) support what he says.
To summarise :-
Cobalamin is stored in liver and kidney. The storage form is not specifically addressed in the review but a more general statement is made that the predominant form of cobalamin in tissues is adenosyl with a small amount of hydroxyl; the form in blood is predominantly methyl. I have seen other studies stating that the storage form is adenosyl. So it seems that adenosyl is the storage form but I would like more direct evidence for this. In any case, the details of storage and release from storage are not understood.
Cobalamin in blood (predominantly the methyl form) is taken into cells bound to the carrier protein transcobalamin 2 (TCN2), via a specific receptor (Quadros was involved in isolation and characterisation of both the carrier protein and the receptor).
The 3-component complex is processed inside the cell with the receptor recycled to the cell surface and cobalamin/TCN2 sent to lysozomes to release the cobalamin from the binding protein.
This is when the first of the errors occurs - (cblF) - cobalamin largely remains stuck in the lysozome.
Normally though free cobalamin with its upper axial ligand attached (ie methyl, adenosyl etc) is released from the lysozome and two steps follow.
The first removes the ligand (the dealkylation reaction described in the reference linked by
@Eastman, which is performed by the gene product of
MMACHC, defined by cblC.
The second, the product of
MMADHC definied by cblD, changes the oxidation state of the cobalt, a cobalamin reductase.
From here, cobalamin is shepherded either to the enzyme complex MTR/MTRR where cobalt is further reduced and a methyl group added, or into the mitochondrion. cblE and G define defects in MTRR and MTR respectively.
Two different variants defined by cblD (variant 1 and 2) appear to act as chaperones in this process, protecting and stabilising cobalamin (ie the cobalamin is never left naked). Variant 1 is involved with MTR/MTRR and variant 2 with MUT.
In the mitochondrion, the product of
MMMA, defined by cblA, of uncertain function, but presumably a cobalamin reductase, further reduces the cobalt atom. Then an adenosyltransferase, the product of
MMMB, defined by cblB, adds an adenosyl group (in an ATP dependant reaction) before the cofactor binds to its enzyme MUT (methyl malonylcoA mutase).
A defect in
MUT is described as MUT rather than a cbl letter.
I have uploaded a diagram.
The other relevant studies were also performed by Quadros. In these, various labelled forms of cobalamin were added to cells and the products analysed. These showed that the three forms studied, viz cyano, adenosyl and methyl, were readily interconverted.
That's all I have the energy for now.
Intracellular processing of cobalamin is very complex and is not fully understood.
Initially though the upper axial ligand is removed from all forms once they are inside the cell. From there, cobalamin can be converted to either of the active forms.
Labelling studies show that the forms can interconvert readily.
Probably adenosyl is the storage form but storage mechanisms are understood even less well.