Vitamin B6 is a water-soluble vitamin that exists in three major chemical forms: pyridoxine, pyridoxal, and pyridoxamine [1,2].
Pyridoxal, pyridoxamine and pyridoxine are collectively known as vitamin B6. All three compounds are efficiently converted to the biologically active form of vitamin B6, pyridoxal phosphate. This conversion is catalyzed by the ATP requiring enzyme, pyridoxal kinase.
Pyridoxal phosphate functions as a cofactor in enzymes involved in transamination reactions required for the synthesis and catabolism of the amino acids as well as in glycogenolysis as a cofactor for glycogen phosphorylase.

Biological Function of Vitamin B6
Vitamin B6 must be obtained from the diet because humans cannot synthesize it, and the coenzyme, PLP plays a vital role in the function of approximately 100 enzymes that catalyze essential chemical reactions in the human body (1, 2). For example, PLP functions as a coenzyme for glycogen phosphorylase, an enzyme that catalyzes the release of glucose stored in the muscle as glycogen. Much of the PLP in the human body is found in muscle bound to glycogen phosphorylase. PLP is also a coenzyme for reactions used to generate glucose from amino acids, a process known as gluconeogenesis.

Pyridoxal phosphate inhibits angiogenesis and down regulates the activity of RNA polymerase, reverse transcriptase and DNA polymerase.
Vitamin B6, through its involvement in protein metabolism and cellular growth, is important to the immune system. It helps maintain the health of lymphoid organs (thymus, spleen, and lymph nodes) that make white blood cells. Animal studies show that a vitamin B6 deficiency can decrease antibody production and suppress the immune response. Vitamin B6 also helps maintain blood glucose (sugar) within a normal range by releasing glucose from glycogen. When caloric intake is low the body needs vitamin B6 to help convert stored carbohydrate or other nutrients to glucose to maintain normal blood sugar levels. While a shortage of vitamin B6 will limit these functions, supplements of this vitamin do not enhance them in well-nourished individuals (1, 8-10).

Signs and Symptoms of Deficiency of Vitamin B6
Clinical signs of vitamin B6 deficiency are rarely seen however, Vitamin B6 deficiency can occur in individuals with poor quality diets that are deficient in many nutrients. Symptoms occur during later stages of deficiency, when intake has been very low for an extended time. Signs of vitamin B6 deficiency include dermatitis, glossitis, depression, confusion, and convulsions (1,11). Vitamin B6 deficiency also can cause anemia (1,12,14) abdominal distress, abnormal electroencephalogram, acne, anorexia, autoimmune diseases.

References:

1. Leklem JE. Vitamin B6. In: Shils ME, Olson JA, Shike M, Ross AC, ed. Modern Nutrition in Health and Disease. 9th ed. Baltimore: Williams and Wilkins, 1999: 413-421.
2. Bender DA. Vitamin B6 requirements and recommendations. Eur J ClinNutr 1989 ;43:289-309. [PubMed abstract].
3. Gerster H. The importance of vitamin B6 for development of the infant. Human medical and animal experiment studies. Z Ernahrungswiss 1996; 35:309-17. [PubMed abstract].
4. Bender DA. Novel functions of vitamin B6. ProcNutrSoc 1994; 53:625-30. [PubMed abstract].
5. Chandra R and Sudhakaran L. Regulation of immune responses by Vitamin B6. NY AcadSci 1990; 585:404-423. [PubMed abstract].
6. Trakatellis A, Dimitriadou A, Trakatelli M. Pyridoxine deficiency: New approaches in immunosuppression and chemotherapy. Postgrad Med J 1997; 73:617-22. [PubMed abstract].
7. Shibata K, Mushiage M, Kondo T, Hayakawa T, Tsuge H. Effects of vitamin B6 deficiency on the conversion ratio of tryptophan to niacin. BiosciBiotechnolBiochem 1995; 59:2060-3.
   [PubMed abstract].
8. Leyland DM and Beynon RJ. The expression of glycogen phosphorylase in normal and dystrophic muscle. Biochem J 1991; 278:113-7. [PubMed abstract].
9. Oka T, Komori N, Kuwahata M, Suzuki I, Okada M, Natori Y. Effect of vitamin B6 deficiency on the expression of glycogen phosphorylase mRNA in rat liver and skeletal muscle. Experientia 1994; 50:127-9. [PubMed abstract].
10. Okada M, Ishikawa K, Watanabe K. Effect of vitamin B6 deficiency on glycogen metabolism in the skeletal muscle, heart, and liver of rats. J NutrSciVitaminol (Tokyo) 1991; 37:349-57. [PubMed abstract].
11. U.S. Department of Agriculture, Agricultural Research Service,1999. USDA Nutrient Database for Standard Reference, Release 13. Nutrient Data Lab Home Page, http://www.nal.usda.gov/fnic/foodcomp.
12. Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes: Thiamin, riboflavin, niacin, vitamin B6, folate, vitamin B12, pantothenic acid, biotin, and choline. NationalAcademy Press. Washington, DC, 1998.
13. Alaimo K, McDowell M, Briefel R, Bischof A, Caughman C, Loria C, and Johnson C. Dietary intake of vitamins, minerals, and fiber of persons ages 2 months and over in the United States: Third National Health and Nutrition Examination survey, Phase 1, 1988-91. Hyattsville, MD: U.S. Department of Health and Human Services; Center for Disease Control and Prevention; NationalCenter for Health Statistics, 1994:1-28.
14. Combs G. The Vitamins: Fundamental aspects in nutrition and health. San Diego, California: Academic Press, Inc., 1992; 311-328.