Vitamin B12 (Cobalamin): Its Fate from Ingestion to Metabolism with Particular Emphasis on Diagnostic Approaches of Acquired Neonatal/Infantile Deficiency Detected by Newborn Screening

Abstract

Acquired vitamin B12 (vB12) deficiency (vB12D) of newborns is relatively frequent as compared with the incidence of inherited diseases included in newborn screening (NBS) of different countries across the globe. Infants may present signs of vB12D before 6 months of age with anemia and/or neurologic symptoms when not diagnosed in asymptomatic state. The possibility of identifying vitamin deficient mothers after their pregnancy during the breastfeeding period could be an additional benefit of the newborn screening. Vitamin supplementation is widely available and easy to administer. However, in many laboratories, vB12D is not included in the national screening program. Optimized screening requires either second-tier testing or analysis of new urine and blood samples combined with multiple clinical and laboratory follow ups. Our scope was to review the physiologic fate of vB12 and the pathobiochemical consequences of vB12D in the human body. Particular emphasis was put on the latest approaches for diagnosis and treatment of vB12D in NBS.

Keywords: cobalamin deficiency; diagnostic approaches; newborn screening; vitamin B12 deficiency.

Figure 1

Route of Cbl from intake to the place of absorption. Dietary Cbl is anchored to the protein transcobalamin-1 (TC1, suggested terminology according to Uniprot search term: P20061 or TCO1_HUMAN, common used name: haptocorrin [8]) either in the mouth (TC1 of salivary gland origin) or already before ingestion, since human breastmilk contains TC1 bound Cbl (TC1-Cbl). Attachment to this protein prevents the damage of the cofactor at the acidic pH of the stomach. In the duodenal lumen pancreatic trypsin partially degrades TC1. Though secreted by the parietal cells of the stomach, intrinsic factor (IF) is only bound to Cbl in the duodenum, after it is released from TC1. Among factors affecting IF synthesis are gastrin, acetylcholine and histamine. The newly formed IF-Cbl complex is transported to the ileum where the apical membrane of the intestinal epithelial cells of the brush-border incorporates it via receptor mediated endocytosis.

Figure 2

From absorption to distribution. The IF-Cbl complex is internalized after binding cubam (which is later recycled to the luminal membrane). In the endosome that is evolving to lysosome, the lowering of the pH makes IF dissociate from Cbl, which is then exported to the cytosol by the LMBD1-ABCD4 (lysosomal cobalamin transport escort protein complex—lysosomal cobalamin transporter ABCD4). Cytosolic free Cbl is either processed to active coenzymes for the enterocyte’s own metabolism or undergoes transcytosis, leaving the cell toward the extracellular matrix via ABCC1(ATP-binding cassette, subfamily C, member 1). Within the bloodstream it either takes the route of storage in the liver by binding TC1 (inactive vB12), or stays capable for uptake and utilization by the cells by binding TC2 (active vB12/holotrans-Cbl (HoloTC)).

Figure 3

Intracellular Cbl processing. After leaving the lysosome via the LMDB1-ABCD4 complex, the Cbl in cytoplasm might be linked to various ligands (X-). During processing by MMACHC, ligand “X-“ is removed and the generated cob(II)alamin is further oxidized by MMADHC. This latter contains an N’-terminal mitochondrial leader sequence and is capable of directing the attached Cbl either to the cytoplasm, or to the mitochondria. In the cytoplasm the complex donates Cbl to MS that becomes capable for receiving the Me-group of CH3-THF and converting HCY to Met. The active form of MeCbl-MS contains cob(I)alamin that might spontaneously oxidize to cob(II)alamin leading to inactive MS. Cob(II)alamin is regenerated by the enzyme Met synthase reductase (MSR). In the mitochondria Cbl in the Cbl-MMADHC complex is processed by MMAB enabling the adenosylation of Cbl. The so-formed AdoCbl connects to MUT. Together with MMAA the complex is capable of the enzymatic transition of MMCoA to succinyl CoA (SuCoA) [12,13]. For explanation of protein symbols see Table 1 below. CH3-THF methyl tetrahydrofolate reductase; OH₂Cbl aquo-Cbl; TCA tricarboxylic acid.

Figure 4

Secondary causes of vB12 deficiency. PPI proton pump inhibitor; H2-R histamine2-receptor; IBD inflammatory bowel disease; HIV human immunodeficiency virus.