维生素-K（血凝和骨代谢中的一个重要因子）主要以来自植物的叶绿醌(PK)的形式存在于饮食中。这种维生素的一种形式，即甲萘醌-4或MK-4，在人类和大鼠的大脑、肾脏和胰腺中有高特异性的组织分布，说明叶绿醌是局部合成的。
现在，催化这种合成的一种酶已被识别出来：“含有1的UbiA异戊烯转移酶” (UBIAD1)是大肠杆菌的一种酶的人类同源物。以前其功能不清楚，尽管它在“施奈德结晶状角膜营养不良”中是一个候选基因。
关于一种人类MK-4酶能够生物合成维生素-K的荷尔蒙活性形式的发现，对于有关人类维生素-K需求及骨健康的研究工作将有参考价值。（生物谷Bioon.com）
生物谷推荐英文摘要：
Nature doi:10.1038/nature09464
Identification of UBIAD1 as a novel human menaquinone-4 biosynthetic enzyme
Kimie Nakagawa,Yoshihisa Hirota,Natsumi Sawada,Naohito Yuge,Masato Watanabe,Yuri Uchino,Naoko Okuda,Yuka Shimomura,Yoshitomo Suhara& Toshio Okanot-okano@kobepharma-u.ac.jp
Vitamin?K occurs in the natural world in several forms, including a plant form, phylloquinone (PK), and a bacterial form, menaquinones (MKs). In many species, including humans, PK is a minor constituent of hepatic vitamin?K content, with most hepatic vitamin?K content comprising long-chain MKs. Menaquinone-4 (MK-4) is ubiquitously present in extrahepatic tissues, with particularly high concentrations in the brain, kidney and pancreas of humans and rats1, 2, 3. It has consistently been shown that PK is endogenously converted to MK-4 (refs 4–8). This occurs either directly within certain tissues or by interconversion to menadione (K3), followed by prenylation to MK-4 (refs 9–12). No previous study has sought to identify the human enzyme responsible for MK-4 biosynthesis. Previously we provided evidence for the conversion of PK and K3 into MK-4 in mouse cerebra13. However, the molecular mechanisms for these conversion reactions are unclear. Here we identify a human MK-4 biosynthetic enzyme. We screened the human genome database for prenylation enzymes and found UbiA prenyltransferase containing 1 (UBIAD1), a human homologue of Escherichia coli prenyltransferase menA. We found that short interfering RNA against the UBIAD1 gene inhibited the conversion of deuterium-labelled vitamin?K derivatives into deuterium-labelled-MK-4 (MK-4-d7) in human cells. We confirmed that the UBIAD1 gene encodes an MK-4 biosynthetic enzyme through its expression and conversion of deuterium-labelled vitamin?K derivatives into MK-4-d7 in insect cells infected with UBIAD1 baculovirus. Converted MK-4-d7 was chemically identified by 2H-NMR analysis. MK-4 biosynthesis by UBIAD1 was not affected by the vitamin?K antagonist warfarin. UBIAD1 was localized in endoplasmic reticulum and ubiquitously expressed in several tissues of mice. Our results show that UBIAD1 is a human MK-4 biosynthetic enzyme; this identification will permit more effective decisions to be made about vitamin?K intake and bone health.
