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transcobalamin-2; TC-2; transcobalamin II; salivary factor TC II; TCII (TCN2, TC2)
Function:
- chief transport protein of methyl-cobalamin (vit B12) from ileum to the liver
- ~1/2 of the TC2/vit B12 complex (holo-TC2) is cleared by the liver
- in plasma, holo-TC2 is the metabolically active form of vit B12
- 6-20% of TC2 is saturated as holo-TC2
Structure:
- belongs to the eukaryotic cobalamin transport proteins family
Compartment: secreted
Expression:
- secreted by hepatocytes
- may be expressped by renal tubular epithelial cells
Polymorphism:
- Pro/Arg-259 polymorphism affects TCN2 plasma concentration & may interfere in vitamin B(12) cellular availability & homocysteine metabolism
Pathology:
- defects in TCN2 are the cause of transcobalamin II deficiency
Pharmacokinetics:
- elimination 1/2life of the holo-TC2 in plasma is 1 hour. [3]
- most of the peripheral holo-TC2 clearance occurs via glomerular filtration followed by tubular uptake in the kidney
- renal tubular epithelium is abundant in TC2 receptors [4]
- animal studies suggest that renal tubules also synthesize TC2, which emerges in the blood stream as new holo-TC2 carrying the reabsorbed cobalamin [4]
- pathologic influences on most of these individual phases in the cycle are unknown
General
transcobalamin
Properties
SIZE: entity length = 427 aa
MW = 48 kD
COMPARTMENT: plasma
MOTIF: signal sequence {1-18}
cysteine residue {C21}
MODIFICATION: cysteine residue {C267}
cysteine residue {C116}
MODIFICATION: cysteine residue {C309}
cysteine residue {C165}
MODIFICATION: cysteine residue {C205}
cysteine residue {C205}
MODIFICATION: cysteine residue {C165}
cysteine residue {C267}
MODIFICATION: cysteine residue {C21}
cysteine residue {C309}
MODIFICATION: cysteine residue {C116}
SECRETED-BY: hepatocyte
Database Correlations
OMIM 275350
UniProt P20062
Pfam PF01122
Entrez Gene 6948
Kegg hsa:6948
References
- Clinical Diagnosis & Management by Laboratory
Methods, J.B. Henry (ed), W.B. Saunders Co.,
Philadelphia, PA. 1991, pg 632
- Cotran et al Robbins Pathologic Basis of Disease,
W.B. Saunders Co, Philadelphia, PA 1989 pg 456
- Harrison's Online, Chapter 107, McGraw Hill, 2002
- Carmel R
Clinical Chemistry. 48:407-409. 2002
- UniProt :accession P20062
- TCN2base; Note: TCN2 mutation db
http://bioinf.uta.fi/TCN2base/