Vitamin Bseveral has the largest and most complex chemical structure of all the vitamins. For this reason cobalamin is the term used to refer to compounds having vitamin B12 activity. Methylcobalamin and 5-deoxyadenosylcobalamin are the forms of vitamin B12 used in the human body (1). The form of cobalamin used in most nutritional supplements and fortified foods, cyanocobalamin, is readily converted to 5-deoxyadenosylcobalamin and methylcobalamin in the body. In mammals, cobalamin is a cofactor for only two enzymes, methionine synthase and L-methylmalonyl-coenzyme A mutase (2).
Cofactor for methionine synthase
Methylcobalamin required on the intent behind the newest folate-oriented chemical, methionine synthase. That it enzyme becomes necessary to your synthesis of the amino acid, methionine, off homocysteine. Methionine therefore is necessary into synthesis regarding S-adenosylmethionine, a beneficial methyl class donor found in of a lot physical methylation responses, like the methylation regarding numerous internet within DNA, RNA, and you will protein (3). Aberrant methylation out of DNA and you may healthy protein, that creates changes in chromatin design and you can gene term, is a common element of cancer tissues. Inadequate https://datingranking.net/escort-directory/columbus-1/ purpose of methionine synthase can lead to an accumulation homocysteine, which was from the increased threat of cardiovascular disease (Figure step one).
Cofactor for L-methylmalonyl-coenzyme Good mutase
5-Deoxyadenosylcobalamin is needed from the chemical that catalyzes the latest conversion process away from L-methylmalonyl-coenzyme A towards succinyl-coenzyme A beneficial (succinyl-CoA), which in turn gets in this new citric acid duration (Contour 2). Succinyl-CoA takes on a crucial role in the production of times off lipids and you may necessary protein and is you’ll need for the synthesis of hemoglobin, the latest oxygen-carrying pigment inside red blood structure (3).
In healthy adults, vitamin B12 deficiency is uncommon, mainly because total body stores can exceed 2,500 ?g, daily turnover is slow, and dietary intake of only 2.4 ?g/day is sufficient to maintain adequate vitamin B12 status (see RDA) (4). In elderly individuals, vitamin B12 deficiency is more common mainly because of impaired intestinal absorption that can result in in B12 deficiency in this population.
Reasons for vitamin B12 insufficiency
Intestinal malabsorption, rather than inadequate dietary intake, can explain most cases of vitamin B12 deficiency (5). Absorption of vitamin B12 from food requires normal function of the stomach, pancreas, and small intestine. Stomach acid and enzymes free vitamin B12 from food, allowing it to bind to R-protein (also known as transcobalamin-1 or haptocorrin), found in saliva and gastric fluids. In the alkaline environment of the small intestine, R-proteins are degraded by pancreatic enzymes, freeing vitamin B12 to bind to intrinsic factor (IF), a protein secreted by specialized cells in the stomach. Receptors on the surface of the ileum (final part of the small intestine) take up the IF-B12 complex only in the presence of calcium, which is supplied by the pancreas (5). Vitamin B12 can also be absorbed by passive diffusion, but this process is very inefficient-only about 1% absorption of the vitamin B12 dose is absorbed passively (2). The prevalent causes of vitamin B12 deficiency are (1) an autoimmune condition known as pernicious anemia, and (2) a disorder called food-bound vitamin B12 malabsorption. Both conditions have been associated with a chronic inflammatory disease of the stomach known as atrophic gastritis.
Atrophic gastritis is thought to affect 10%-30% of people over 60 years of age (6). The condition is frequently associated with the presence of autoantibodies directed toward stomach cells (see Pernicious anemia) and/or infection by the bacteria, Helicobacter pylori (H. pylori) (7). H. pylori infection induces chronic inflammation of the stomach, which may progress to peptic ulcer disease, atrophic gastritis, and/or gastric cancer in some individuals. Diminished gastric function in individuals with atrophic gastritis can result in bacterial overgrowth in the small intestine and cause food-bound vitamin B12 malabsorption. Vitamin B12 levels in serum, plasma, and gastric fluids are significantly decreased in individuals with H. pylori infection, and eradication of the bacteria has been shown to significantly improve vitamin B12 serum concentrations (8).