Phylloquinone epoxide

Fats, oils, and dairy products digest sample with lipase, extract with hexane. Purify hexane extracts obtained from either extraction technique by silica solid-phase extraction in the sample cleanup mode. The internal standard is phylloquinone 2,3-epoxide (unlabeled for UV detection and tritium labeled for coulometric detection). 

Fig. 14 Analytical HPLC of the phylloquinone fraction from an extracted sample of brown rice isolated by semipreparative HPLC. Column, Spherisorb C8 (octyl) mobile phase, methanol/50 mM acetate buffer pH 3.0 (97 3) containing 0.1 mM EDTA, dual-electrode coulometric detection (redox mode), porous graphite electrodes, — 1.5 V (generator electrode), +0.05 V (detector electrode). The arrows signify the fraction containing tritiated phylloquinone 2,3-epoxide (internal standard) and phylloquinone (analyte) that is collected for quantitation by radioisotopic dilution. (Courtesy of M. J. Shearer.)...

After a tracer dose of radioactive phylloquinone, the label is rapidly accumulated in the liver, then lost from the body with turnover time of 1.5 days. Tbis suggests that there is rapid turnover and little storage of vitamin K. However, there maybe considerable enterohepatic recirculation of the conjugates excreted in the bile (Shearer et al., 1996 Olson et al., 2002). About 10% of the total liver vitamin K is normally present as the epoxide, which is formed by the vitamin K-dependent carboxylase and normally reduced back to tbe active vitamin (Section 5.3.1). 

Fig. 45.5. A. Structures of vitamin K derivatives. Phylloquinone is found in green leaves, and intestinal bacteria synthesize menaquinone. Humans will convert menadione to a vitamin K active form. B. Vitamin K-dependent formation of y-carboxyglutamate residues. Thrombin, Factor VII, Factor IX, and Factor X are bound to their phospholipid activation sites on cell membranes by Ca ". The vitamin K-dependent carboxylase, which adds the extra carboxyl group, uses a reduced form of vitamin K (KH2) as the electron donor and converts vitamin K to an epxjx-ide. Vitamin K epoxide is reduced, in two steps, back to its active form by the enzymes vitamin K epoxide reductase and vitamin K reductase.

Figure 1 Structural formulas of some K vitamers (1) vitamin Ki(20), (VKl), phylloquinone or 2-methyl-3-eicosa-2 -ene-l,4-naphthoquinone (2) trideuterium labeled VKl, VKl-rfj, 2-methyl-rf3-3-eicosa-2 -ene-1,4-naphthoquinone as internal standard for GC-MS analysis (3) VKl-epoxide (4) menaquinone-4, MK4, 2-methyl-3-tetraprenyl-l,4-naph-thoquinone, as an example of the MKn series (5) VKl-heptafluorobutyryl ester after reductive acylation of VKl (6) menadione, vitamin K3 (VK3), or 2-methyl-1,4-naphthoqui-none (7) menadione sodium bisulfite (MSB), water-soluble form of VK3.

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