Vitamin B-6

Description

Vitamin B-6 (pyridoxine) is a component of the water-soluble vitamin B-Complex vitamins. It is composed of three closely related chemical compounds: pyridoxine, pyridoxal and pyridoxamine. These three compounds are metabolically and functionally interrelated and are readily interconverted.

Vitamin B-6 is involved in protein, fat, carbohydrate metabolism, and is a component of the molecular configuration of many enzymes. Vitamin B-6 functions primarily in the reactions involved in the nonoxidative degradation of amino acids.

Method of Action

Vitamin B-6 is found in cells in the active form as coenzyme pyridoxol phosphate, as well as in its amino form, pyridoxamine phosphate.

Vitamin B-6 participates in transamination. Vitamin B-6 catalyzes the reaction by acting as a carrier of the amino group. During transamination, pyridoxal phosphate accepts an amino group from an amino acid, and is subsequently converted to pyridoxamine phosphate. The amino group is then transferred to a receptor molecule (usually alpha-keto glutarate), and pyridoxal phosphate is regenerated. Transamination reactions can occur between any amino acid and a receptive alpha-keto acid.

Other metabolic processes catalyzed by vitamin B-6 are: deamination; decarboxylation, the removal of carbon dioxide from amino acids, a necessary step required for the synthesis of neurotransmitters from certain amino acids; and desulfuration, the transfer of a sulfur group from methionine to serine in order to form cysteine. All three processes utilize vitamin B-6 as a carrier compound. Vitamin B-6 is also involved in the formation of niacin from tryptophan.

Vitamin B-6 is an important enzyme in the biosynthesis of hemoglobin and in the production of gamma-aminobutyric acid (GABA) from glutamic acid.

Vitamin B-Complex, vitamin B-1, vitamin B-2, vitamin C, pantothenic acid, magnesium, potassium, linoleic acid, and sodium assist in the absorption of vitamin B-6. Absorption is decreased by tobacco, ethanol, coffee, the use of oral contraceptives, and exposure to radiation; it is destroyed by cooking.

Properties & Uses

Vitamin B-6 is used to treat anemic patients when the anemia is due to a vitamin B-6 deficiency.

Vitamin B-6 supplements can help correct the depression, malaise, glucose intolerance and increased tryptophan excretion in some women using oral contraceptives.

Treatment with large doses of vitamin B-6 may prevent the effects of isoniazid, a chemotherapeutic agent for tuberculosis. Vitamin B-6 doses may prevent isoniazid from inhibiting the conversion of glutamic acid, the only amino acid the brain metabolizes.

Consequence of Deficiency

Deficiency of vitamin B-6 can cause seizures by inhibiting the synthesis of gamma-aminobutyric acid (GABA). GABA is a calming chemical and seizures can result if it is present in insufficient amounts.

Vitamin B-6 deficiency has been shown to increase urinary oxalate excretion. This represents the body's inability to convert glyoxalate to glycine, which is necessary to synthesize glycine and serine.

Deficiency in vitamin B-6 can be created by use of oral contraceptives, with subsequent symptoms including depression and increased urinary excretion of tryptophan.

Toxicity Factors

The toxicity of vitamin B-6 is extremely low. Sleepiness may follow an injection of large doses (100 milligrams).

Recommended Dietary Allowance

age	RDA (mg)	RNI (mg)
infants/children
0-6 months	0.3	0.2
7-9 months	0.6	0.3
10-12 months	0.6	0.4
1-3 years	1.0	0.7
4-6 years	1.1	0.9
7-10 years	1.4	1.0
males
11-14 years	1.7	1.2
15-18 years	2.0	1.5
19-50 years	2.0	1.4
51+ years	2.0	-
females
11-14 years	1.4	1.0
15-18 years	1.5	1.0
19+ years	1.6	1.0
pregnancy	2.2	-
lactation	2.1	-

A deficiency of vitamin B-6 is unlikely because, relative to the requirement, the amounts present in the general diet are more than sufficient. Since pyridoxine is involved in amino acid metabolism, the need for pyridoxine varies with dietary protein intake. For adults, approximately one milligram daily is minimal. The RDA standard is two milligrams per day for adults to ensure a safety margin for variances in individual need.

For over thirty years, Recommended Daily Amounts has existed in the United Kingdom. It has been used to measure the adequacy of an individual's diet. However, in 1991 the Committee on Medical Aspects of Food Policy (COMA) gave forth a whole new set of figures upon the request of the Department of Health's Chief Medical Officer. Reference Nutrient Intake (RNI) is one of these sets collectively known as "Dietary Reference Values." RNI is an amount of a nutrient that is enough for almost every individuals, even someone who has high needs for the nutrient. This level of intake is, therefore, considerably higher than what most people would need. If individuals are consuming the RNI of a nutrient they are most unlikely to be deficient in that nutrient.

Food Sources

High: (1,000 - 10,000 mg/100 g)

Beef liver	Blackstrap molasses
Brewer's yeast	Brown rice
Herring	Peanuts
Pork liver	Salmon
Veal liver	Walnuts
Wheat germ

Medium: (100 - 1,000 mg/100 g)

Avocado	Bananas
Barley	Butter
Cabbage	Carrots
Cod	Corn
Egg	Flounder
Grapes	Green peas
Halibut	Kale
Mackerel	Oat flakes
Organ meats	Pears
Potatoes	Rye bread
Sardines	Soybeans
Tuna	Whale
Whole wheat

Low: (10 - 100 mg/100 g)

Apples	Asparagus
Beans	Beet greens
Black currants	Cantaloupe
Cheese	Cherries
Grapefruit	Lemon
Lettuce	Milk
Onion	Orange
Peach	Raisins
Strawberries	Watermelon

Abstracts

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