Vitamin B6 Functions

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. 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.

Nervous system function

The synthesis of the neurotransmitter, serotonin, from the amino acid, tryptophan, in the brain is catalyzed by a PLP-dependent enzyme. Other neurotransmitters such as dopamine, norepinephrine and gamma-aminobutyric acid (GABA) are also synthesized using PLP-dependent enzymes.

Red blood cell formation and function

PLP functions as a coenzyme in the synthesis of heme, a component of hemoglobin. Hemoglobin is found in red blood cells and is critical to their ability to transport oxygen throughout the body. Both PL and PLP are able to bind to the hemoglobin molecule and affect its ability to pick up and release oxygen. However, the impact of this on normal oxygen delivery to tissues is not known (2).

Niacin formation

The human requirement for another vitamin, niacin, can be met in part by the conversion of the dietary amino acid, tryptophan, to niacin, as well as through dietary intake. PLP is a coenzyme for a critical reaction in the synthesis of niacin from tryptophan. Thus, adequate vitamin B₆ decreases the requirement for niacin in the diet.

Hormone function

Steroid hormones, such as estrogen and testosterone, exert their effects in the body by binding to steroid hormone receptors in the nucleus of the cell and altering gene transcription. PLP binds to steroid receptors in such a manner as to inhibit the binding of steroid hormones, thus decreasing their effects. The binding of PLP to steroid receptors for estrogen, progesterone, testosterone, and other steroid hormones suggest that the vitamin B₆ status of an individual may have implications for diseases affected by steroid hormones, such as breast cancer and prostate cancer.

Nucleic acid synthesis

PLP serves as a coenzyme for a key enzyme involved in the mobilization of single-carbon functional groups (one-carbon metabolism). Such reactions are involved in the synthesis of nucleic acids. The effect of B₆ deficiency on immune system function may be partly related to the role of PLP in one-carbon metabolism.