Vitamin B6
Vitamin B6 is a water-soluble vitamin that is naturally present in many foods, added to others, and available as a dietary supplement. It is absorbed in the jejunum. Vitamin B6 is the generic name for six compounds, that are collectively called vitamers. The six compounds include pyridoxine, pyridoxal, pyridoxamine and their respective 5’-phosphate esters. Pyridoxal-5-phosphate and pyridoxamine-5-phosphate are cofactors in more than 100 enzyme reactions. They are especially important in amino acid metabolism. Vitamin B6 also plays a role in cognitive development through the biosynthesis of neurotransmitters and in maintaining normal levels of homocysteine. Vitamin B6 is involved in gluconeogenesis and glycogenolysis, immune function, and hemoglobin formation.
Patients with end stage renal disease, those undergoing dialysis and renal transplant recipients often have low vitamin B6 concentrations. Rheumatoid arthritis is often associated with low B6 concentrations, possibly due to increased inflammatory cytokines. Patients with celiac disease, Crohn’s disease, ulcerative colitis, inflammatory bowel disease, and other malabsorptive autoimmune disorders tend to have low plasma pyridoxal-5-phosphate concentrations. People with alcoholism tend to have low levels.
Some antiepileptic drugs, including valproic acid (Depakene®, Stavzor®), carbamazepine (Carbatrol®, Epitol®, Tegretol®, and others), and phenytoin (Dilantin®) increase the catabolism rate of vitamin B6 vitamers, resulting in low plasma PLP concentrations. Other medications that may reduce the levels of vitamin B6 include: cycloserine, isoniazid, hydralazine, penicillamine, theophylline, pyrazinamide, levodopa, and carbidopa.
The primary reason for ordering vitamin B6 is to diagnose pyridoxine deficiency, which has been associated with microcytic hypochromic anemia, dermatitis, neuritis, stomatitis and cheliosis. Chronic B6 deficiency can lead to secondary hyperoxaluria and an increased risk of kidney stones. Chronic B6 deficiency has also been associated with nerve compression disorders such as carpal tunnel and tarsal tunnel syndromes, but this relationship is controversial.
Markedly elevated levels of pyridoxal-5-phosphate are observed in hypophosphatasia, a disorder characterized by skeletal deformaties and llow alkaline phosphatase levels. High levels are also detected in patients taking megadoses of pyridoxine (>2g per day). Megadose therapy may cause peripheral neuropathy.
Pyridoxal-5-phosphate is the vitamer that is measured in serum or plasma to determine vitamin B6 level. Reference range is 5 to 50 ug/L. The Food and Nutrition Board at the Institute of Medicine of the National Academies used a plasma pyridoxal-5-phosphate level of 5 ug/L (20 nmol/L) as the major indicator of adequacy to calculate the Recommended Dietary Allowances (RDAs) for adults.
Specimen requirement is one red top or green top tube of blood.
