四烯甲萘醌

四烯甲萘醌，又称维他命K₂，为一群相似的化合物，可分为短链四烯甲萘醌（如MK-4）和长链四烯甲萘醌（如MK-7、MK-8、MK-9）。维生素K₂未有已知的毒性，因维生素K不会在肝脏中过量残留。

构造

维生素K₂

维生素K皆有醌结构，而维生素K₂的命名取决于上图化学式中n的数目。例如当n为4，名称就是MK-4。维生素K₂于身体中的传输会受其结构影响。

维生素K的结构，当中MK-4、MK-7均为维生素K₂

维生素K循环

作用机制

维生素K₂的作用机制与维生素K₁类似，能够辅助γ-谷氨酰羧化酶（英语：Gamma-glutamyl carboxylase）将谷氨酸（Glu）转化成γ-羧化谷氨酸（Gla）。这过程对生成Gla-蛋白质（含有Gla的蛋白质）有帮助，例子如下：

骨钙素：骨骼矿化
GAS6：细胞增殖
凝血酶、抗凝蛋白：血液稳态

吸收

维生素K于小肠中被吸收，藉乳糜微粒运输。维生素K₁和MK-4能够被肝脏快速处理，但低密度脂蛋白（LDL）会带走较多长链四烯甲萘醌。由于LDL寿命较长，长链四烯甲萘醌能够被肝以外的组织（骨、软骨、血管等）吸收，对达致维生素K部分功效有帮助。^[1]

摄入

除了肝脏外，蕴含长链四烯甲萘醌最丰富的是以细菌发酵的食品，如乳酪（MK-8、MK-9）、纳豆（MK-7）等，当中纳豆为维生素K₂含量最丰富的食品^[2]。根据对荷兰人饮食习惯的研究，估计维生素K的摄取由约 90% 维生素K₁、约 7.5 % MK-5至MK-9及约 2.5% MK-4组成。另外根据对完全不饱和四烯甲萘醌的研究，乳酪含有10–20 μg/100g 的MK-8和35–55 μg/100 g 的MK-9。^[3]

另外人体肠道中的细菌亦可制造维生素K₂，可以满足每日对维生素K的部分需求。

抗凝剂及维生素K的补充

研究指出长期口服抗凝剂减少体内正常骨钙素，从而影响骨骼健康。这可能导致骨骼矿化不足（骨质减少）、容易骨折等^[4]。另外长期口服抗凝剂亦相信会令小童、成人出现不良的软组织钙化^[5]^[6]，这是由于缺乏维生素K导致正常基质Gla蛋白偏少。在动物实验中血管钙化可出现于服用华法林两周内^[7]。另外长期口服抗凝剂的病人被发现其动脉钙化的程度为其他病人的一倍^[8]^[9]。动脉钙化亦可能导致收缩期高血压、心室肥大等^[10]^[11]。虽然大量维生素K降低抗凝剂的效用，但现在正研究将抗凝剂疗程及维生素K结合。

参考文献

^ Martin J. Shearer, Paul Newman. Metabolism and cell biology of vitamin K. Thromb Haemost 2008
^ Kaneki M, Hodges SJ, Hosoi T, Fujiwara S, Lyons A, Crean SJ, Ishida N, Nakagawa M, Takechi M, Sano Y, Mizuno Y, Hoshino S, Miyao M, Inoue S, Horiki K, Shiraki M, Ouchi Y, Orimo H; Japanese fermented soybean food as the major determinant of the large geographic difference in circulating levels of K vitamins2: possible implications for hip-fracture risk; Nutrition; 2001; 17(4): 315-321.
^ Shearer N, Metabolism and cell biology of vitamin K. Thromb Haemost. 2008
^ Caraballo PJ, Gabriel SE, Castro MR, Atkinson EJ, Melton LJ 3rd. Changes in bone density after exposure to oral anticoagulants: a meta-analysis.Osteoporos Int. 1999;9(5):441-8.
^ Barnes C, Newall F, Ignjatovic V, Wong P, Cameron F, Jones G, Monagle P. Reduced bone density in children on long-term warfarin. Pediatr Res. 2005;57(4):578-81.
^ Hawkins D, Evans J. Minimising the risk of heparin-induced osteoporosis during pregnancy. Expert Opin Drug Saf. 2005;4(3):583-90
^ Price PA, Faus SA, Williamson MK. Warfarin causes rapid calcification of the elastic lamellae in rat arteries and heart valves. Arterioscler Thromb Vasc Biol. 1998;18(9):1400-7.
^ Schurgers LJ, Aebert H, Vermeer C, Bültmann B, Janzen J. Oral anticoagulant treatment: friend or foe in cardiovascular disease? Blood. 2004 15;104(10):3231-2.
^ Koos R, Mahnken AH, Mühlenbruch G, Brandenburg V, Pflueger B, Wildberger JE, Kühl HP. Relation of oral anticoagulation to cardiac valvular and coronary calcium assessed by multislice spiral computed tomography. Am J Cardiol. 2005;96(6):747-9.
^ Zieman SJ, Melenovsky V, Kass DA. Mechanisms, pathophysiology, and therapy of arterial stiffness. Arterioscler Thromb Vasc Biol. 2005;25(5):932-43.
^ Raggi P, Shaw LJ, Berman DS, Callister TQ. Prognostic value of coronary artery calcium screening in subjects with and without diabetes. J Am Coll Cardiol. 2004;43(9):1663-9.

外部链接

Vitak BV
VitaminK2.org（页面存档备份，存于互联网档案馆）

[1] Martin J. Shearer, Paul Newman. Metabolism and cell biology of vitamin K. Thromb Haemost 2008

[2] Kaneki M, Hodges SJ, Hosoi T, Fujiwara S, Lyons A, Crean SJ, Ishida N, Nakagawa M, Takechi M, Sano Y, Mizuno Y, Hoshino S, Miyao M, Inoue S, Horiki K, Shiraki M, Ouchi Y, Orimo H; Japanese fermented soybean food as the major determinant of the large geographic difference in circulating levels of K vitamins2: possible implications for hip-fracture risk; Nutrition; 2001; 17(4): 315-321.

[3] Shearer N, Metabolism and cell biology of vitamin K. Thromb Haemost. 2008

[4] Caraballo PJ, Gabriel SE, Castro MR, Atkinson EJ, Melton LJ 3rd. Changes in bone density after exposure to oral anticoagulants: a meta-analysis.Osteoporos Int. 1999;9(5):441-8.

[5] Barnes C, Newall F, Ignjatovic V, Wong P, Cameron F, Jones G, Monagle P. Reduced bone density in children on long-term warfarin. Pediatr Res. 2005;57(4):578-81.

[6] Hawkins D, Evans J. Minimising the risk of heparin-induced osteoporosis during pregnancy. Expert Opin Drug Saf. 2005;4(3):583-90

[7] Price PA, Faus SA, Williamson MK. Warfarin causes rapid calcification of the elastic lamellae in rat arteries and heart valves. Arterioscler Thromb Vasc Biol. 1998;18(9):1400-7.

[8] Schurgers LJ, Aebert H, Vermeer C, Bültmann B, Janzen J. Oral anticoagulant treatment: friend or foe in cardiovascular disease? Blood. 2004 15;104(10):3231-2.

[9] Koos R, Mahnken AH, Mühlenbruch G, Brandenburg V, Pflueger B, Wildberger JE, Kühl HP. Relation of oral anticoagulation to cardiac valvular and coronary calcium assessed by multislice spiral computed tomography. Am J Cardiol. 2005;96(6):747-9.

[10] Zieman SJ, Melenovsky V, Kass DA. Mechanisms, pathophysiology, and therapy of arterial stiffness. Arterioscler Thromb Vasc Biol. 2005;25(5):932-43.

[11] Raggi P, Shaw LJ, Berman DS, Callister TQ. Prognostic value of coronary artery calcium screening in subjects with and without diabetes. J Am Coll Cardiol. 2004;43(9):1663-9.

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