Mol. Nutr. Food Res. 2014, 58, 1589
Editorial: Maret G. Traber
1589
fat-soluble vitamin. Moreover, the absorption, transport, and metabolism of the various forms of vitamin K are not well understood. The vitamin is converted from PK to MK-4 in some but not all tissues, and the vitamin is aggressively recycled, making studies of its metabolism a challenge, as reviewed by Card et al. They emphasize two problem areas in vitamin K research: 1) its interaction with vitamin E, where both Hanzawa et al and Farley et al provide some novel insights into the interactions of these two vitamins; and 2) vitamin K water-soluble metabolites, compounds which are present at the urine at even lower concentrations. Soper et al provide some clues to potential therapeutic roles of these metabolites in treatment of osteoporosis. In addition to its role in blood clotting and vascular Vitamin K, known as the “Koagulation” vitamin, was calcification, an important role of vitamin K is in the discovered in 1929 because deficiency of this vitamin maintenance of bone density. Hamidi and Cheung caused bleeding in rats; it is now recognized to be a review the randomized controlled trials of vitamin K critical co-factor for the gamma-carboxylation of vitamin supplementation in osteoporosis, but conclude that K-dependent proteins. The 17 known vitamin more research is needed. K-dependent proteins regulate calcium in the Despite vitamin K’s low circulating concentrations, blood-clotting cascade, artery walls, bone matrix and the importance of vitamin K-dependent proteins should brain to mention a few sites of action. The mechanism not be minimized. Willems et al point out in their review for vitamin K function is well known. Vitamin K [either that vitamin K has emerged as a “multi-function phylloquinone (PK, the dietary form) or menaquinone-4 vitamin”, where the critical role of vitamin K is artery (MK-4, the tissue form)] is used by vitamin K wall calcification is now apparent, especially with regard carboxylase (GGCX) to carboxylate glutamic acids (Glu) to matrix Gla protein (MGP). Rafael et al provide to ␥-carboxyglutamic acids (Gla) in vitamin exciting new data concerning Gla-rich protein (GRP), a K-dependent proteins. Gla are necessary for the vitamin K-dependent protein characterized by large calcium-chelating function of vitamin K-dependent number of Gla resides. This protein accumulates in proteins. Thus, it is now widely appreciated that vitamin sites of vascular calcification in humans. Much of the K is essential for calcium regulation! available information suggests Importantly, vitamin K recycling is that vitamin K is a key player in critical for its function. Dietary vitamin “ . . . it is now widely control of calcium, but there K must be reduced to the remains much research to do in appreciated that vitamin K is K-hydroquinone, which is then oxidized the emerging field of vitamin K essential for calcium by GGCX to the K-epoxide during Gla and vitamin K-dependent regulation!” formation. The K-epoxide is then proteins. reduced in two steps, first it is reduced to the K-quinone, and next to the K-hydroquinone. This recycling allows each vitamin K molecule to participate in hundreds of carboxylation reactions. This issue of Molecular Nutrition & Food Research focuses on “Vitamin K Conundrums”, specifically new or challenging areas for research. Studies of this fat Maret G. Traber soluble vitamin are challenging because circulating Linus Pauling Institute concentrations of vitamin K are exceptionally low, more Oregon State University than 10,000 times lower than vitamin E, another Corvallis, OR, USA
Vitamin K Conundrums
C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.mnf-journal.com
Editorial: Maret G. Traber
1589
fat-soluble vitamin. Moreover, the absorption, transport, and metabolism of the various forms of vitamin K are not well understood. The vitamin is converted from PK to MK-4 in some but not all tissues, and the vitamin is aggressively recycled, making studies of its metabolism a challenge, as reviewed by Card et al. They emphasize two problem areas in vitamin K research: 1) its interaction with vitamin E, where both Hanzawa et al and Farley et al provide some novel insights into the interactions of these two vitamins; and 2) vitamin K water-soluble metabolites, compounds which are present at the urine at even lower concentrations. Soper et al provide some clues to potential therapeutic roles of these metabolites in treatment of osteoporosis. In addition to its role in blood clotting and vascular Vitamin K, known as the “Koagulation” vitamin, was calcification, an important role of vitamin K is in the discovered in 1929 because deficiency of this vitamin maintenance of bone density. Hamidi and Cheung caused bleeding in rats; it is now recognized to be a review the randomized controlled trials of vitamin K critical co-factor for the gamma-carboxylation of vitamin supplementation in osteoporosis, but conclude that K-dependent proteins. The 17 known vitamin more research is needed. K-dependent proteins regulate calcium in the Despite vitamin K’s low circulating concentrations, blood-clotting cascade, artery walls, bone matrix and the importance of vitamin K-dependent proteins should brain to mention a few sites of action. The mechanism not be minimized. Willems et al point out in their review for vitamin K function is well known. Vitamin K [either that vitamin K has emerged as a “multi-function phylloquinone (PK, the dietary form) or menaquinone-4 vitamin”, where the critical role of vitamin K is artery (MK-4, the tissue form)] is used by vitamin K wall calcification is now apparent, especially with regard carboxylase (GGCX) to carboxylate glutamic acids (Glu) to matrix Gla protein (MGP). Rafael et al provide to ␥-carboxyglutamic acids (Gla) in vitamin exciting new data concerning Gla-rich protein (GRP), a K-dependent proteins. Gla are necessary for the vitamin K-dependent protein characterized by large calcium-chelating function of vitamin K-dependent number of Gla resides. This protein accumulates in proteins. Thus, it is now widely appreciated that vitamin sites of vascular calcification in humans. Much of the K is essential for calcium regulation! available information suggests Importantly, vitamin K recycling is that vitamin K is a key player in critical for its function. Dietary vitamin “ . . . it is now widely control of calcium, but there K must be reduced to the remains much research to do in appreciated that vitamin K is K-hydroquinone, which is then oxidized the emerging field of vitamin K essential for calcium by GGCX to the K-epoxide during Gla and vitamin K-dependent regulation!” formation. The K-epoxide is then proteins. reduced in two steps, first it is reduced to the K-quinone, and next to the K-hydroquinone. This recycling allows each vitamin K molecule to participate in hundreds of carboxylation reactions. This issue of Molecular Nutrition & Food Research focuses on “Vitamin K Conundrums”, specifically new or challenging areas for research. Studies of this fat Maret G. Traber soluble vitamin are challenging because circulating Linus Pauling Institute concentrations of vitamin K are exceptionally low, more Oregon State University than 10,000 times lower than vitamin E, another Corvallis, OR, USA
Vitamin K Conundrums
C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.mnf-journal.com
