Vitamin B6 assessment and toxicity; vitamin B12 sources and initial absorption
THE CONTEXT
This passage describes functional assessment and excess intake of vitamin B6, then introduces vitamin B12 chemistry, dietary availability, and binding proteins.
Vitamin B6 status and excess
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The most widely used assessment measures activation of erythrocyte transaminases after pyridoxal phosphate is added in vitro; the result is expressed as an activation coefficient. Plasma vitamin concentrations are also measured.
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Sensory neuropathy has been reported with pyridoxine intakes of 2–7 g daily, with some residual damage after withdrawal. Other reports associate intakes exceeding 100–200 mg daily with neurological damage.
Vitamin B12 identity and sources
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Vitamin B12 is a generic descriptor for cobalamins: cobalt-containing corrinoids with a corrin ring and the biological activity of the vitamin. Some other corrinoids support microbial growth but lack vitamin B12 activity and may act as antimetabolites.
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Vitamin B12 is synthesized exclusively by microorganisms. For practical dietary purposes it occurs only in foods of animal origin, placing strict vegetarians at risk of deficiency.
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Small amounts produced by bacteria on fruit surfaces may meet requirements. Preparations made by bacterial fermentation are available.
Cobalt coordination
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Four cobalt coordination sites bind nitrogen atoms of the corrin ring, and another binds nitrogen of the dimethylbenzimidazole nucleotide.
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The sixth site distinguishes cyanocobalamin, hydroxocobalamin, aquocobalamin, methylcobalamin, and adenosylcobalamin.
Binding proteins in absorption
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Gastric acid and pepsin release vitamin B12 from food proteins, allowing it to bind cobalophilin, a protein secreted in saliva.
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Cobalophilin is hydrolyzed in the duodenum, releasing the vitamin for binding to intrinsic factor.
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Intrinsic factor is a small glycoprotein secreted by gastric parietal cells; vitamin B12 is absorbed bound to it.