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Pyridoxal phosphate (PLP) is the active form of vitamin B6. It is also known as pyridoxal-5-phosphate or P5P. This phosphate is a combination of pyridoxal, pyridoxamine, and pyridoxine — all of which are natural forms of vitamin B6. PLP is not a protein, but acts as a co-factor for a number of enzymes and protein groups. A co-factor, also known as a prosthetic group, is a chemical compound which is bound to, and required for, a protein to function.
One of the main functions of pyridoxal phosphate is as a coenzyme in transamination reactions. Transamination is a key process in the formation of non-essential amino acids, and would not be possible without the presence of PLP. This reaction is considered to be kinetically perfect, which means that every time it can occur, it does. These types of reactions are uncommon, and some display kinetics that are quicker than the rate of diffusion, which should be chemically impossible. Speculation as to the mechanism by which this process operates so efficiently include dipolar electric fields, and the quantum-mechanical tunneling of protons.
Vitamin B6 is part of the B-complex vitamin group, and is water soluble. It was first isolated in the 1930s in rats undergoing nutrition studies. In 1934, Hungarian physician Paul Gyorgy discovered that this same compound was able to mitigate skin diseases in rats, and he named it vitamin B6. Over the next ten years, vitamin B6 was isolated in rice bran; all three precursor forms of pyridoxal phosphate were discovered; and vitamin B6 was given the name pyridoxine.
PLP is essential in a wide variety of enzymatic activities. The British Enzyme Commission (EC) has documented over 140 enzyme reactions that depend on pyridoxal phosphate to function. This is equal to approximately 4% of all known enzyme reactions.
In total, there are seven known varieties of vitamin B6, but only PLP is metabolically active. All of them can be converted into one another, however, except for 4-pyridoxic acid. Once this version of the vitamin is formed, it is excreted in the urine.
Pyridoxal phosphate is also involved in macronutrient metabolism, neurotransmitter synthesis, and gene expression. In addition, it also plays a role in gluconeogenesis (GNG), which is one of two processes the human body uses to maintain glucose levels in the body. GNG is a metabolic pathway, and PLP reactions in the body provide the necessary amino acids for the pathway to create glucose. This form of the B6 vitamin is one of the most functional and highly-used in the human body.