B12 Lab Report

Chapter

Vitamin B12 Absorption and Transport in Human Body

Omar Abuyaman, MSc

Department of Clinical Biochemistry, Aarhus University hospital, Aarhus, Denmark.

Abstract

Mammals are unable to synthesize B12. Instead, they have a sophisticated multistep pathway for specific and efficient transport of this vitamin from its food source to the target body cells. Dysfunction at any of transport steps may lead to low vitamin B12 status or deficiency.

Introduction

The transport of B12 from its food source to reach the body cells is mediated by a complex set of carrier proteins, receptors and transporters [1]. The selective multistep pathway of B12 transport, include the liberation of B12 from its matrix in the ingested food, gastrointestinal transport,

The released B12 subsequently binds HC [12]. this binding is thought to shield the vitamin from chemical modification or hydrolysis by the acidic environment in the stomach. (Del Corral and Carmel 1990; Kittang and Schjonsby 1987) gastric dysfunction and diminished acid secretion (gastric atrophy, gastric surgery or treatment with acid suppressing drugs) may lead to B12 malabsorption. Different foods have different vitamin B12 bioavailability (the amount of vitamin B12 available for absorption) mainly depend on the ability of the stomach environment to liberate the vitamin from them. As example vib12 from the fish meat is less bioavailability than the B12 from the milk, mainly because of the limited ability for stomach conditions to liberate the bound B12 from its

Cubam, on the other hand, recycles to the cell surface. Intrinsic factor is degraded by actions of lysosomal protease (most likely by cathepsin L enzyme which differ from the intestinal proteases) in the lysosome, [56].. and the Cbl is released, (inhibitors of lysosomal function block secretion of Cbl from cultured opossum kidney cells) [55]. Released vitamin B12 traverses the lysosomal membrane and enters the cytoplasm, a process that probably involves the protein LMBD1 and ABCD4. LMBD1 is a 61 kDa lipocalin receptor-like protein locate to the lysosomal membrane while the ABCD4 is ATP-binding cassette (ABC) transporter [41]. Mutations in the gene encoding LMBD1 or ABCD4 are responsible for the rare inborn defects named cblF and cblJ respectively with similar phenotype and clinical symptoms. Defects in LMBD1 or ABCD4 resulting in trapping of free vitamin B12 in lysosomes and therefore fail to absorb oral vitamin B12 [41] [42] . [43]. The mechanism of LMBD1 and ABCD4 contribution in the intracellular transport of vitamin B12 is currently unclear. However, because ABCD4 may co-localize with LMBD1 in the lysosomes it was proposed that the translocation mediated by LMBD1 and regulated by ABCD4. How B12 is handled within the cell from the point of lysosomal exit until its usage as a coenzyme or its export from the