Vitamin B12 absorption: Mammalian physiology and acquired and inherited disorders

Background

Vitamin B12 (cobalamin) is a cobalt-containing compound synthesized by bacteria and an essential nutrient in mammals, which take it up from diet. The absorption and distribution of dietary vitamin B12 to the organism is a complex process involving several gene products including carrier proteins, plasma membrane receptors and transporters. Disturbed cellular entry, transit or egress of vitamin B12 may lead to low vitamin B12 status or deficiency and eventually hematological and neurological disorders.

Objective

The aim of this review is to summarize the causes leading to vitamin B12 deficiency including decreased intake, impaired absorption and increased requirements. Under physiological conditions, vitamin B12 bound to the gastric intrinsic factor is internalized in the ileum by a highly specific receptor complex composed by Cubilin (Cubn) and Amnionless (Amn). Following exit of vitamin B12 from the ileum, general cellular uptake from the circulation requires the transcobalamin receptor CD320 whereas kidney reabsorption of cobalamin depends on Megalin (Lrp2).Whereas malabsorption of vitamin B12 is most commonly seen in the elderly, selective pediatric, nondietary-induced B12 deficiency is generally due to inherited disorders including the Imerslund-Gräsbeck syndrome and the much rarer intrinsic factor deficiency. Biochemical, clinical and genetic research on these disorders considerably improved our knowledge of vitamin B12 absorption.This review describes basic and recent findings on the intestinal handling of vitamin B12 and its importance in health and disease.     

Role of Third Serum Vitamin B12 Binding Protein in Vitamin B12 Transport

A third vitamin B₁₂ binding protein present in normal serum has been shown to participate in transport of labelled vitamin B₁₂ absorbed from the gut. All three vitamin B₁₂ binding proteins in serum were labelled at the same time after oral administration of vitamin B₁₂, implying that “free” vitamin B₁₂ reached the portal blood from the gut mucosa.     

Vitamin B12 formation from various actinomycetes isolated from bulgarian soils

240 зтаммов актиномицетов, выделенных из ризосферы различных растений и из различных почв, исследовались на наличие витамина В₁₂ путем культивирования в жидкой питательной среде. 73% из исследованных штаммов обладают способностью образовывать витамин В₁₂, в количестван порядка 0,0025–1,6 μ/мл. Количество продуцентов зависит от растительного покрова. Серия Helvolus включает наибольшее количество продуцентов (93,3%).