Transcobalamin II Mediated Delivery of Albumin-Bound Hydroxocobalamin to Human Liver Cells

Abstract

We show that hydroxocobalamin bound to human serum albumin can dissociate and bind to transcobalamin II present in serum. Human liver cells in culture exposed to hydroxocobalamin bound to albumin incorporated less of the vitamin than when similar amounts of unbound hydroxocobalamin or cyanocobalamin were present. In the presence of transcobalamin II, a 4.5-fold increase in cellular uptake occurred, but this amount was less than when hydroxocobalamin or cyanocobalamin were added to transcobalamin II. These results indicate that albumin, by binding hydroxocobalamin, can alter the dynamics of binding to transcobalamin II and the subsequent cellular incorporation of this form of the vitamin.

Investigation of Receptor-Mediated Cyanocobalamin (Vitamin B12) Transport across the Inner Blood–Retinal Barrier Using Fluorescence-Labeled Cyanocobalamin

“…Vitamin B 12 is involved in various cellular physiological events, such as one-carbon metabolism including folate and methionine metabolism pathways that are associated with DNA methylation. ,, While the intestinal absorption of vitamin B 12 has been suggested to involve several molecules, such as IF, haptocorrin, cubilin, megalin, and ASGP-R, − the tissue distribution of vitamin B 12 involves TCII and TCII-R. − In the neural retina, vitamin B 12 has a role that is associate to retinal pathological symptoms, and its property as a superoxide scavenger has been also reported. − However, little is known about the details of the mechanisms for supplying vitamin B 12 to the retina across the BRB. In the present study, the importance of TCII and TCII-R in the retinal distribution of vitamin B 12 was hypothesized, and the in vivo and in vitro visual assessment of cyanocobalamin transport at the BRB was performed, by means of Cy5-cyanocobalamin synthesized successfully (Figure ).…”

“…The membrane transport of vitamin B 12 has been investigated in various types of cells, and the previous studies have suggested the expression of cubilin, megalin (low density lipoprotein receptor-related protein 2; LRP2), and asialoglycoprotein receptor (ASGP-R) in the intestinal epithelial cells. − Intrinsic factor (IF) secreted by the gastric epithelium is involved in the intestinal absorption of vitamin B 12 . The IF-vitamin B 12 complex binds to cubilin in the ileal brush-border, and the interaction between cubilin and megalin is suggested to be crucial for the endocytosis of the IF-vitamin B 12 complex. − In addition, ASGP-R expressed in hepatocytes or intestinal epithelial cells is suggested to contribute to the uptake of vitamin B 12 formed complex with haptocorrin (transcobalamin I). − Regarding the tissue distribution of vitamin B 12 from the circulating blood, in vitro transport studies in various human-derived cells, such as skin fibroblasts, erythroleukemia (K562 cells), colorectal adenocarcinoma (SW48 cells), and embryonic kidney cell line (HEK293 cells), have suggested the involvement of transcobalamin II receptor (TCII-R/CD320/TCblR)-mediated endocytosis in the blood-to-tissue transport of vitamin B 12 . − Therefore, vitamin B 12 is required to form a complex with TCII protein, and the complex associates with TCII-R at the cell surface, followed by internalization mediated by clathrin-dependent endocytosis. Subsequently, the digestion of the complex by cathepsin L takes place in the lysosomes, and vitamin B 12 is freed to be transported from lysosomes to the cytosol by the ATP-biding cassette transporter subfamily D member 4 (ABCD4). , The acidic lysosomal interior is essential for the stability and activity of cathepsin L, and it has been suggested that the neutralization of lysosomal pH by lysosomotropic agents including chloroquine inhibits intracellular vitamin B 12 transport. , Based on TCII/TCII-R pathway, vitamin B 12 analogues such as [ 111 indium]-diethylenetriaminepentaacetate adenosylcobalamin ( 111 In-DAC) was synthesized to be an imaging tool for TCII-R with an implication that it can delineate tumors in their locations. − …”

The prevalence and treatment of metformin associated Vitamin B12 deficiency in patients with type 2 Diabetes Mellitus in New Zealand

“…This transport of cobalamin from the blood and into an intracellular storage state may explain the large discrepancy between the measured amount cobalamin excretion and the serum Vitamin B12 level after injection with Hydroxocobalamin. It has also been suggested that there is preferential movement of cobalamin to liver storage by TC-II in the presence of serum albumin when the cobalamin is in the form of Hydroxocobalamin (Begley, Colligan, & Chu, 1993). This may suggest a higher preferential storage and slow usage of Vitamin B12 in the Hydroxocobalamin form, this would also aid in explanation of the lower response peak of Vitamin B12 in the serum but a longer effect tail as the vitamin is used.…”

“…It has been suggested that metformin increases liver preferential storage of Vitamin B12 (Greibe, Miller, et al, 2013), coupled with the higher affinity of Hydroxocobalamin to bind to hepatic parenchymal cells via preferential trafficking (Begley et al, 1993), may suggest that the decreased serum Vitamin B12 level response to Hydroxocobalamin compared to Methylcobalamin is due to increased hepatic storage of the vitamin as opposed to a more deficient state or lower total Vitamin B12 in the body.…”