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VITAMINS & MINERALS / 491VITAMIN B 6 IS IMPORTANT IN AMINOACID & GLYCOGEN METABOLISM& IN STEROID HORMONE ACTIONSix compounds have vitamin B 6 activity (Figure 45–12):pyridoxine, pyridoxal, pyridoxamine, and their 5′-phosphates. The active coenzyme is pyridoxal 5′-phosphate.Approximately 80% of the body’s total vitamin B 6is present as pyridoxal phosphate in muscle, mostly associatedwith glycogen phosphorylase. This is not availablein B 6 deficiency but is released in starvation, when glycogenreserves become depleted, and is then available, especiallyin liver and kidney, to meet increased requirementfor gluconeogenesis from amino acids.Vitamin B 6 Has Several Rolesin MetabolismPyridoxal phosphate is a coenzyme for many enzymesinvolved in amino acid metabolism, especially intransamination and decarboxylation. It is also the cofactorof glycogen phosphorylase, where the phosphategroup is catalytically important. In addition, vitamin B 6is important in steroid hormone action where it removesthe hormone-receptor complex from DNAbinding, terminating the action of the hormones. In vitaminB 6 deficiency, this results in increased sensitivityto the actions of low concentrations of estrogens, androgens,cortisol, and vitamin D.HOCH 2HOCH 2HOCH 2CH 2 OHNOHCH 3PyridoxineHC=ONPyridoxalOHCH 3CH 2 NH 2OHNCH 3PyridoxamineKINASEPHOSPHATASEKINASEPHOSPHATASEOOOPOCH 2CH 2 OHOHOCH 3Pyridoxine phosphateOPOCH 2AMINOTRANSFERASESKINASEPHOSPHATASEONHC=OOHOCH 3Pyridoxal phosphateOPOCH 2NOXIDASECH 2 NH 2OHOCH 3Pyridoxamine phosphateFigure 45–12. Interconversion of the vitamin B 6vitamers.NOXIDASEVitamin B 6 Deficiency Is RareAlthough clinical deficiency disease is rare, there is evidencethat a significant proportion of the populationhave marginal vitamin B 6 status. Moderate deficiencyresults in abnormalities of tryptophan and methioninemetabolism. Increased sensitivity to steroid hormone actionmay be important in the development of hormonedependentcancer of the breast, uterus, and prostate,and vitamin B 6 status may affect the prognosis.Vitamin B 6 Status Is Assessed by AssayingErythrocyte AminotransferasesThe most widely used method of assessing vitamin B 6status is by the activation of erythrocyte aminotransferasesby pyridoxal phosphate added in vitro, expressedas the activation coefficient.In Excess, Vitamin B 6 CausesSensory NeuropathyThe development of sensory neuropathy has been reportedin patients taking 2–7 g of pyridoxine per day fora variety of reasons (there is some slight evidence that itis effective in treating premenstrual syndrome). Therewas some residual damage after withdrawal of these highdoses; other reports suggest that intakes in excess of 200mg/d are associated with neurologic damage.VITAMIN B 12 IS FOUND ONLYIN FOODS OF ANIMAL ORIGINThe term “vitamin B 12 ” is used as a generic descriptorfor the cobalamins—those corrinoids (cobalt containingcompounds possessing the corrin ring) havingthe biologic activity of the vitamin (Figure 45–13).Some corrinoids that are growth factors for microorganismsnot only have no vitamin B 12 activity but mayalso be antimetabolites of the vitamin. Although it issynthesized exclusively by microorganisms, for practicalpurposes vitamin B 12 is found only in foods of animalorigin, there being no plant sources of this vitamin.This means that strict vegetarians (vegans) are atrisk of developing B 12 deficiency. The small amountsof the vitamin formed by bacteria on the surface offruits may be adequate to meet requirements, butpreparations of vitamin B 12 made by bacterial fermentationare available.Vitamin B 12 Absorption Requires TwoBinding ProteinsVitamin B 12 is absorbed bound to intrinsic factor, asmall glycoprotein secreted by the parietal cells of the