Menadione biological activity

Vitamin K3 — menadione — is the only form isolated from Staphylococcus aureus and also chemically synthesized. It is a synthetic compound that can be converted into K2 in the gastrointestinal tract. Unlike other fat-soluble vitamins obtained by chemical synthesis, vitamin K3 is characterized by its high biological activity, just like the naturally occurring ones. 

Assuming therefore that only vitamin K substances with a typical side chain, as in vitamin Ki and the forms of vitamin Kj, are active in metabolism, the above-mentioned differences in the biological activity of vitamin Ki and of menadione can be explained. Menadiones, necessary only in very small doses to cure nutritional vitamin K deficiency, may be quantitatively transformed into the active vitamin Ka form. This bio thesis, being limited, however, as described in Section VI, is insufficient to provide the high amount of metabolically active vitamin Kj needed to counteract Dicumarol poisoning. 

Metabolism. Whether the vitamin Ks functions are unchanged or are transformed to other metabolically active forms has not been determined. It is known that menadione must be converted to Kj in the bodies of both animals and humans for it to be biologically active. 

Coupling between a biologically catalyzed reaction and an electrochemical reaction, referred to as bioelectrocatalysis, is the constructional principle for enzyme-based electrochemical biosensors. This means that the flow of electrons from a donor through the enzyme to an acceptor must reach the electrode in order for the corresponding current to be detected. In case a direct electron transfer between the active site of an enzjane and an electrode is not possible, a small molecular redox active species, e.g. hydrophobic ferrocene, meldola blue and menadione as well as hydrophilic ferricyanide, can be used as an electron transfer mediator. This means that the electrons from the active site of the enzyme reduce the mediator molecule, which, in turn, can diffuse to the electrode, where it donates the electrons upon oxidation. When these mediator molecules are employed for coupling of an enzymatic redox reaction to an electrode at a constant potential, the resulting application can be referred to as mediated amperometry or mediated bioelectrocatalysis. 

Biological samples may contain any or all of three forms of vitamin K vitamin Ki(20) (phylloquinone) which is synthesized by green plants and found in chloroplasts of photosynthetic plants and is the major form of vitamin K in plant and most animal foods vitamin K2, a group of vitamers (menaqui-nones) synthesized by bacteria and ranging from MK-4 to MK-13 according to the number of isoprene units in the side chain, and finally synthetic vitamin K3 (menadione), which exhibits vitamin K activity by virtue of its in vivo conversion to menaquinones, chiefly MK-4. The latter is a pharmaceutical and water-soluble form which does not occur in food and unlike the other forms is toxic at high intakes. Such water-soluble derivatives of menadione are used as animal feed supplement but owing to toxicity to humans menadione cannot be used in human medicine or as a food supplement. 

---