Vitamin B12 and Folate Notes

Vitamin B12 and folate are critical water-soluble vitamins that play essential roles in various physiological processes, particularly in DNA synthesis and red blood cell production.
Deficiencies in these vitamins can lead to macrocytic anemia, characterized by the presence of large red blood cells, which can impact overall health and function.

Food Sources: Rich sources of Vitamin B12 include eggs, meat (particularly red meat), poultry, shellfish (like clams and fish), milk, and dairy products. Fortified cereals and non-dairy milk alternatives are also significant sources for individuals with dietary restrictions.
Vitamin B12 is primarily obtained through dietary intake in humans, being synthesized by bacteria found in the animal gut. As such, vegans and individuals with malabsorption issues are at a higher risk of deficiency.
The general daily requirement for Vitamin B12 in a mixed diet is approximately $5 ext{ } ext{μg}$ , although individual requirements may vary based on age, pregnancy, and overall health status.

The structural composition of Vitamin B12 consists of a planar corrin ring, which houses a central cobalt ion.
It also includes a ribonucleotide (which plays a role in its metabolism) and 5,6-dimethyl benzimidazole, contributing to its biological activity.

The absorption of Vitamin B12 in the intestines is facilitated by several factors, with R-binder haptocorrin binding to B12 in the stomach and protecting it until it reaches the terminal ileum where absorption occurs.
Intrinsic factor, a glycoprotein secreted by the parietal cells of the stomach, is critical for the proper absorption of Vitamin B12.

Molecular Weight: $57 ext{ kDa}$
It is synthesized in the gastric parietal cells. Its main role is to bind Vitamin B12 and protect it from degradation as it travels through the digestive system to the small intestine for absorption. This transport is essential for Vitamin B12's bioavailability and function within the body.

In the bloodstream, Vitamin B12 is bound to two plasma proteins known as transcobalamins:
- TC I: Holds a high affinity for Vitamin B12, serving mostly as a storage form.
- TC II: More readily donates Vitamin B12 to tissues where it is needed, associated with haptocorrin (also known as holotranscobalamin).

The primary storage site for Vitamin B12 is in the liver, where it is maintained in substantial quantities (3-5 mg).
The rate of loss of Vitamin B12 from the body's stores is approximately 0.05-0.1% per day, allowing for dietary deficiencies to be compensated for up to two years or more.

Folate, a B-vitamin, can be found in both plant (like leafy greens, legumes, and fruits) and animal sources (such as liver and eggs).
The average western diet provides about 400 mg of folate, while the recommended adult intake ranges from 100-200 mg/day, with increased needs during pregnancy or lactation due to fetal development requirements.

The molecular structure of folate consists of several components including pyrimidine, pyrazine, 6-methylpterin, p-aminobenzoic acid, and glutamate.
Pteroic acid, along with pteroylglutamic acid (folic acid), are crucial for its synthesis and biological functions in the body.

Dietary folates exist as polyglutamates. In the intestinal lumen, they are converted to monoglutamate by the action of a conjugase enzyme, specifically (\gamma)-glutamyl hydrolase.
To enter the portal bloodstream, 5-methyltetrahydrofolate monoglutamate must be present, ensuring a sufficient supply for cellular activities.
The body reserves of folate are typically less than three months, necessitating regular dietary intake to maintain plasma levels.

Following the conversion of dietary folates (Polyglutamates) to Folate (Monoglutamate) in the intestines, the liver and other tissues further process this form into biologically active derivatives including:- Dihydrofolate (DHF)
- Tetrahydrofolate (THF)
- 5,10-Methylenetetrahydrofolate (5,10-METHF)
- 5-Methyltetrahydrofolate (5-MTHF)
- Falinic Acid
Key enzymes that drive these transformations include:- Methylenetetrahydrofolate Reductase (MTHFR)
- Pyridoxal 5'-Phosphate (P5'P)
- Riboflavin 5'-Phosphate (R5'P)

Causes:
- Inadequate intake, especially in individuals adhering to vegan diets without supplementation.
- Impaired secretion of intrinsic factor (IF) due to conditions like pernicious anemia, total/partial gastrectomy, or rare congenital deficiencies.
- Impaired release of Vitamin B12 from food caused by gastrectomy or gastric dysfunction; the acidic environment is essential for the release of bound B12 from food.
- Diversion of Vitamin B12 due to issues like abnormal intestinal flora, intestinal strictures, stagnated loops, or infections such as in fish tapeworms (especially in Finland).

Symptoms of deficiency can manifest as:
- A red and beefy tongue caused by epithelial cell atrophy.
- Fatigue, often described as lethargy, can indicate anemia and related deficits.
- Neurological symptoms such as paresthesia, or pins and needles in extremities, arising from nerve damage.

Causes:
- Inadequate dietary intake, often seen in those with poor nutrition.
- Malabsorption syndromes such as Coeliac disease, Crohn’s disease, or other gastrointestinal disorders.
- Increased physiological demands during pregnancy, in premature infants, or in malignancies where cellular turnover is heightened.

Vitamin B12: Exists in two functional forms that are necessary for metabolic processes:
- Methyl cobalamin: Essential for the conversion of homocysteine to methionine, which is vital for protein synthesis and DNA methylation.
- Adenosyl cobalamin: Required for the metabolism of certain fatty acids and amino acids, involved in energy production.

The conversion of homocysteine to methionine utilizes both methyl cobalamin and 5-methyl THF, which generates tetrahydrofolate (THF), a crucial cofactor in various biochemical pathways.
Adenosyl cobalamin is critical for the conversion of methylmalonyl-CoA to succinyl-CoA, an important step in lipid metabolism, which is catalyzed by mutase and is essential for heme synthesis.

Both Vitamin B12 and folate are required for the synthesis of thymidine, a fundamental building block of DNA.
Deficiency in either nutrient can result in the substitution of uracil for thymidine during DNA replication, which significantly hampers the accuracy of DNA synthesis and repair, leading to slower maturation of erythrocytes (red blood cells).