Clarification of the conversion mechanism of vitamin K analogues to menaquinone-4, a potent hormone-like substance, in the rat tissues.

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Menaquinone-4 (MK-4) is one of naturally occurring form of vitmin K_2 (MK-n or K2) which is majorly synthesized in the microorganisms, but recently it has been shown that MK-4 is also synthesized from other vitamin K analogues in various tissues of rat and possibly in other mammalian tissues. According to recent investigations undertaken by us and other American and Dutch researchers, MK-4 abundantly distributes than other VK analogues to various tissues in the rats, such as testis, lung, brain, submandibullar gland, heart, spleen, liver, pancreas, and kidney. Recent results also showed that MK-4 is one of potent accelerator of bone formation in the rats, so MK-4 has been thought to be a potent hormone-like substance.We have newly established an MK-4 formation system in vitro using various tissues of the rats. Substrates for this reaction system are vitamin K_3 (menadione or VK3) or vitamin K_1 (phylloquinone or VK1) as a source for naphthoquinone ring, and geranylgeranyl-diphosphate a … More s a source for isoprenyl side-chain of MK-4, respectively. Supernatant fraction of 900 g of rat kidney homogenate with phophate buffer (pH 7.4) was used as crude enzyme solution. The present experiment undertaken for 2 years revealed that MK-4 production from VK3 was inhibited in a dose-related manner under the presence of warfarin that is an inhibitor for VKs epoxide and VKs reductases. This inhibition was recovered completely by an addition of reducing agent, di-thiothreitol (DTT). This lead us to consider that VK reductase participate in the conversion reaction from VK3 to MK-4, and that VK3-H_2 (reduced form of VK3) and MK-4-H_2 (reduced form of MK-4) might be involved in the reaction pathway as an intermediates. Purification trials of this key enzyme is now going on, and we are observing that the dissociation from the microsomal or mitochondrial membrane fraction made this enzyme much more unstable (less active).We have partly clarified the conversion mechanism of VKs into MK-4 much more clearly than before, and got useful information to purify this enzyme ([MK-4 synthase] as a tentative name). Further investigations are necessary to understand the physiological role of accumulated MK-4 in various tissues in the rat. Less