TRANSPORT OF VITAMIN B₁₂ IN MAN

Recognition of two intracellular cobalamin binding proteins and their identification as methylmalonyl-CoA mutase and methionine synthetase.

“…The subsequent plasma transport of Cbl (3) is mediated by transcobalamin II, which delivers Cbl to various tissues (4), and by a granulocyte derived R-type cobalamin binding protein, which delivers Cbl exclusively to hepatocytes (5) via a mechanism that is capable of clearing various asialoglycoproteins (6). Several studies (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21) (35).…”

Absence of an intracellular cobalamin-binding protein in cultured fibroblasts from patients with defective synthesis of 5′-deoxyadenosylcobalamin and methylcobalamin

“…Based on in mvo experiments with radioisotopically labeled Cbl in man (1)(2)(3), it has been shown that, once absorbed from the small intestine, the vitamin is transported in blood tightly bound to a specific serum protein, transcobalamin II (TC II). The subsequent delivery of Cbl to hepatocytes (3)(4)(5)(6)(7) and other tissue cells (8)(9)(10)(11)(12)(13)) is mediated by a complicated, still incompletely understood process which is initiated by the binding of the TC II-Cbl complex to specific cell surface receptors. Current evidence favors the belief that the membrane-bound TC II-Cbl complex then enters the cell intact via endocytosis, and is localized transiently in secondary ly- (26).…”

Serum B₁₂ Binders in Vitamin B₁₂ Deficiency

“…Re lease of TC I from liver may be another possibility. Vitamin B,, is mainly stored in the liver [27] probably bound to a protein with /5-mo bility [28], The plasma B12 is a part of the major vitamin B12 pool in tissues [27], and TC I is responsible for transport of endogenous Bi2 out of the cell [29]. It may well be that, with prolonged deprivation of vitamin B12 and in presence of anaemia, the release of endogenous B12 into the circulation may be accompanied with unsaturated TC I and leads to its moderate increase in serum.…”