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Detection and Quantification of Vitamin K<Sub>1</Sub> Quinol in Leaf Tissues

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Detection and Quantification of Vitamin K<Sub>1</Sub> Quinol in Leaf Tissues University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Agronomy & Horticulture -- Faculty Publications Agronomy and Horticulture Department 2008 Detection and quantification of vitamin K1 quinol in leaf tissues Chloë van Oostende University of Nebraska–Lincoln, [email protected] Joshua R. Widhalm University of Nebraska–Lincoln, [email protected] Gilles J.C. Basset University of Nebraska-Lincoln, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/agronomyfacpub Part of the Plant Sciences Commons van Oostende, Chloë; Widhalm, Joshua R.; and Basset, Gilles J.C., "Detection and quantification of vitamin K1 quinol in leaf tissues" (2008). Agronomy & Horticulture -- Faculty Publications. 588. https://digitalcommons.unl.edu/agronomyfacpub/588 This Article is brought to you for free and open access by the Agronomy and Horticulture Department at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Agronomy & Horticulture -- Faculty Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Published in Phytochemistry 69:13 (October 2008), pp. 2457–2462; doi: 10.1016/j.phytochem.2008.07.006 Copyright © 2008 Elsevier Ltd. Used by permission. Submitted April 4 2008; revised May 21 2008; accepted July 16 2008; published online September 15 2008. Detection and quantification of vitamin 1K quinol in leaf tissues Chloë van Oostende, Joshua R. Widhalm, and Gilles J.C. Basset Center for Plant Science Innovation, University of Nebraska–Lincoln, Lincoln, NE 68588, USA Corresponding author — G.J.C. Basset, tel 402 472-2930, fax 402 472-3139, email [email protected] Authors van Oostende and Widhalm contributed equally to this work. Abstract Phylloquinone (2-methyl-3-phytyl-1,4-naphthoquinone; vitamin K1) is vital to plants. It is responsible for the one- electron transfer at the A1 site of photosystem I, a process that involves turnover between the quinone and semi-qui- none forms of phylloquinone. Using HPLC coupled with fluorometric detection to analyze Arabidopsis leaf extracts, we detected a third redox form of phylloquinone corresponding to its fully reduced – quinol–naphthoquinone ring (PhQH2). A method was developed to quantify PhQH2 and its corresponding oxidized quinone (PhQ) counterpart in a single HPLC run. PhQH2 was found in leaves of all dicotyledonous and monocotyledonous species tested, but not in fruits or in tubers. Its level correlated with that of PhQ, and represented 5–10% of total leaf phylloquinone. Analysis of purified pea chloroplasts showed that these organelles accounted for the bulk of PhQH2. The respective pool sizes of PhQH2 and PhQ were remarkably stable throughout the development of Arabidopsis green leaves. On the other hand, in Arabidopsis and tomato senescing leaves, PhQH2 was found to increase at the expense of PhQ, and repre- sented 25–35% of the total pool of phylloquinone. Arabidopsis leaves exposed to light contained lower level of PhQH2 than those kept in the dark. These data indicate that PhQH2 does not originate from the photochemical reduction of PhQ, and point to a hitherto unsuspected function of phylloquinone in plants. The putative origin of PhQH2 and its recycling into PhQ are discussed. Keywords: Phylloquinone, Vitamin K1, Naphthoquinone, Quinone, Quinol, Chlororespiration, Photosystem I Abbreviations: Gla, γ-carboxyglutamate; MK-4, menaquinone-4; MK-4H2, menaquinone-4 quinol form; PhQ, phyllo- quinone quinone form; PhQH2, phylloquinone quinol form 1. Introduction and vertebrates posses an enzyme that is capable of reducing the naphthoquinone 1 ring to its quinol 3 form (Chu et al., Phylloquinone (2-methyl-3-phytyl-1,4-naphthoquinone) 1 or 2006). Vitamin K-synthesizing organisms, on the other hand, vitamin K1, is a conjugated isoprenoid consisting of a naphtho- use vitamin K 1 as an electron transporter. In photosynthetic quinone ring attached to a C-20 phytyl side chain (Figure 1A). organisms, for instance, phylloquinone 1 is the electron car- It is synthesized exclusively by plants, algae, and cyanobacte- rier at the A1 site of photosystem I. There, it transfers a sin- ria. Facultative anaerobic bacteria synthesize a closely related gle electron at a time from chlorophyll A to the iron–sulfur form called menaquinone (vitamin K2). Vertebrates, that can- center of ferredoxin reductase (Sigfridsson et al., 1995). This not synthesize vitamin K, require it as an essential micronutri- one-electron transfer thus involves the quinone 1 and semi- ent. For humans, plant-based foods represent the main dietary quinone 2 forms of phylloquinone (Figure 1B), and typically source of this vitamin (Damon et al., 2005). occurs in the nanosecond time-scale. The quinone/semiqui- Vitamin K 1 plays very different roles in the organisms none (1:2) turnover is classically studied by electron para- that cannot synthesize it compared to those that can. In ver- magnetic resonance (Xu et al., 2003), while the comparatively tebrates, for instance, it serves as a cofactor for certain car- stable quinone 1 can be analyzed by HPLC coupled with flu- boxylases that convert specific glutamate residues in target orometric or electrochemical detection (Booth and Sadowski, proteins into γ-carboxyglutamates (Gla) (Ulrich et al., 1988). 1997; McCarthy et al., 1997). Such Gla-containing proteins are involved in blood coagula- Taking advantage of the sensitivity and selectivity of tion, bone homeostasis, and the maintenance of vascular in- HPLC–fluorometry, we report here the existence of the fully tegrity (Vermeer et al., 2004). The biologically active form of reduced form of phylloquinone 3 (PhQH2) in leaf tissues. After the vitamin required for such processes is vitamin K quinol 3, showing that PhQH2 3 is localized in chloroplast, we surveyed i.e. the one with a fully reduced naphthoquinone moiety (Fig- its respective pool sizes and its oxidized quinone counter- ure 1B). The homeostasis of vitamin K quinol 3 is therefore part 1 (PhQ) in Arabidopsis leaves of different developmental key to the function of the vitamin K-dependent carboxylases, stages, and subjected to light and dark conditions. 2457 2458 VAN OOSTENDE, WIDHALM, & BASSET IN PHYTOCHEMISTRY 69 (2008) thione, tocopherols) released from the tissue during its homog- enization. It was especially a concern since it had been shown that another conjugated plant quinone, plastoquinone, could be reduced nonspecifically in ethanolic solution by ascorbate (Kruk and Karpinski, 2006). Therefore, as a control experiment we spiked the Arabidopsis leaf tissue before homogenization with 450 pmol of menaquinone-4 (MK-4; vitamin K2). MK-4 is not synthesized by plants; however, it contains the same naph- thoquinone ring as phylloquinone, and thus displays simi- lar redox properties. Due to its structurally different polyiso- prenyl side chain, MK-4 can also be easily distinguished from phylloquinone 1 by reversed phase analysis. After extraction and HPLC separation, while PhQH23 was detected, MK-4H2 was not (data not shown). Nor did we detect any PhQH23 when pure PhQ 1 was mixed prior to extraction with ascorbate (30 mM), conditions that are known to cause significant reduc- tion of plastoquinone (Kruk and Karpinski, 2006). From these control experiments, it can be concluded that the detection of PhQH23 in plant extracts was not caused by the non-specific reduction of PhQ 1 during tissue homogenization. In addition, the tissues were spiked at the beginning of the extraction with Figure 1. Structure and redox forms of phylloquinone 1 (vitamin K ). 1 an internal standard consisting of a mixture of MK-4H2/MK- (A) The phytyl side-chain of phylloquinone 1 consists of one isopen- 4, whose ratio had been quantified. By comparing the initial tenyl unit followed by three isopentyl units. (B) Interconversion be- MK-4H /MK-4 ratio with that measured in the sample after tween the quinone 1 (oxidized), semiquinone 2 (hemi-reduced), and 2 quinol 3 (fully reduced) forms of phylloquinone 1. The latter is known extraction and HPLC separation, the respective PhQH23 and only in vertebrates, and appears to originate from the direct enzymatic PhQ 1 levels could be corrected for reoxidation of the naph- bi-reduction of the quinone. R = phytyl. thoquinone moiety. The same internal standard was used for the calculation of the recovery of total phylloquinone (PhQ + 2. Results and discussion PhQH2). Reoxidation levels ranged from 5% to 40%, and total recovery varied from 76% to 98%. 2.1. Detection and quantification of PhQH23 in leaf tissues PhQH23 was readily detected in the leaves of all dicotyle- donous and monocotyledonous species examined (Table 1). Its Usual methods for the quantification of phylloquinone 1 in level correlated with that of PhQ 1, and was found to repre- plant samples involve the preparation of a lipid-enriched frac- sent ~5% to 10% of the total pool of leaf PhQ 1. On the other tion prior to analysis. These classically include extraction in an hand, in tubers and fruits, whose PhQ 1 content was very low, organic solvent, phase partitioning, evaporation of an organic PhQH23 was below the detection limit (~0.01 pmol/mg FW) if phase and sample resolubilization. Pilot experiments using Ara- present at all in these organs. bidopsis leaves showed that HPLC–fluorometry provided suf- ficient selectivity and sensitivity to render these pre-treatments 2.2. PhQH23 is mainly—if not exclusively—localized in unnecessary, and that conjugated naphthoquinones could be di-
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