Vitamin B₆, Plasma

Vitamin B₆ occurs as an alcohol (pyridoxine), an aldehyde (pyridoxal), and an amine (pyridoxamine). These forms are phosphorylated in the 5'-position to produce the physiologically active coenzymes that are critical to their biological function. Eukaryotes cannot synthesize vitamin B₆ molecules from smaller compounds and as a result require dietary B₆ for the synthesis of 5'-phosphate vitamins. Pyridoxal 5'Phosphate (PLP), the most clinically significant coenzyme form of vitamin B₆, is the form most commonly measured in plasma.^1-3

PLP serves as a coenzyme for more than 100 enzymes that catalyze key steps in the metabolism of amino acids, neurotransmitters, nucleic acids, heme, and lipids.^1,4,5 Vitamin B₆ is a critical cofactor for enzymes involved in energy homeostasis through glycogen degradation and gluconeogenesis.⁵ Inverse associations have been shown between plasma PLP and chronic or acute disease, including rheumatoid arthritis, cardiovascular disease, deep vein thrombosis, and cancer.^4-16 A number of epidemiologic studies have shown reduced concentrations of circulating PLP in association the acute phase marker C-reaction protein^13-17 and with inflammatory markers.^18-19 Diminished vitamin B₆ levels are frequently observed without any indication of a lower dietary intake or excessive catabolism of the vitamin, or congenital defects in its metabolism.⁴ Research is ongoing to determine if these lower vitamin B₆ levels are caused by the mobilization of this coenzyme to the site of inflammation for use by the PLP-dependent enzymes⁴ or due increased catabolism of vitamin B₆ during inflammation.⁵

PLP serves as a coenzyme for δ-aminolevulinate synthase, which catalyzes the first step in heme biosynthesis.^1,5 B₆ deficiency can produce a hypochromic form of anemia characterized by the presence of ring sideroblasts (iron positive granules deposited about the nucleus of red cell precursors). Occasionally the anemia may have megaloblastic characteristics. Inherited abnormalities of apoenzymes that bind with pyridoxal phosphate are responsible for newborn conditions characterized by mental retardation, skeletal deformities, thrombotic conditions, osteoporosis, and visual defects. Some inherited abnormalities of vitamin B₆ metabolism and transport are associated with aminoacidurias including homocystinuria, hypermethioninemia, cystathioninuria.²¹ A number of studies have demonstration an inverse association between plasma PLP levels and the risk of developing colorectal cancer.²⁰ A recent meta- analysis indicated that the risk of developing this type of cancer decreased by 49% for every 100-pmol/mL increase in blood PLP level.²⁰

Vitamin B₆ deficiency can occur in individuals with a variety of genetic conditions including antiquitin deficiency,²¹ pyridox(am)ine-5'-phosphate oxidase (PNPO) deficiency²² and hyperprolinemia type II (pyrroline-5- carboxylate dehydrogenase deficiency.²³ Vitamin B₆ levels can be decreased in malabsorption conditions including inflammatory disease of the small bowel and as a consequence of jejunoileal bypass.^4,5 Several drugs, including oral contraceptive agents, levodopa, isoniazid, cycloserine, and pyrazinoic acid may cause B₆ depletion.¹ B₆ levels may be decreased with pregnancy, lactation and alcoholism.¹ Infants can develop deficiency when fed formula rendered B₆ depleted by excessive heating.

Markedly elevated plasma PLP levels are observed in cases of hypophosphatasia (HPP), an inborn error of metabolism caused by a loss-of-function mutation(s) within the gene for the cell surface enzyme, tissue nonspecific isoenzyme of alkaline phosphatase (TNSALP).^24-28 This disorder is characterized by low serum alkaline phosphatase activity and increased plasma levels of TNSALP substrates including inorganic pyrophosphate, phosphatidylethanolamine and PLP. Clinical features can include childhood rickets, adult osteomalacia and dental abnormalities. These symptoms are thought to occur as a result of the accumulation of inorganic pyrophosphate which inhibits hydroxyapatite crystal formation and growth, leading to defective skeletal and dental mineralization. PLP, carried in the plasma on albumin, must be de-phosphorylated by TNSALP for pyridoxal to cross cell membranes. Once inside the cell, the pyridoxal is regenerated as PLP to allow it to function as a coenzyme. The diminished TNSALP of individuals with HPP leads to an accumulation of the PLP substrate in plasma. HPP patients do not typically experience B₆ related symptoms. However, the extent of PLP elevation has been related to the disease severity.²⁸