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Determination of Individual Vitamin B6 Compounds Based on Enzymatic Conversion to 4-Pyridoxolactone

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Determination of Individual Vitamin B6 Compounds Based on Enzymatic Conversion to 4-Pyridoxolactone J Nutr Sci Vitaminol, 54, 18–24, 2008 Determination of Individual Vitamin B6 Compounds Based on Enzymatic Conversion to 4-Pyridoxolactone Saki NISHIMURA, Sayaka NAGANO, Chan A. CRAI, Nana YOKOCHI, Yu YOSHIKANE, Fei GE and Toshiharu YAGI* Department of Bioresources Science, Faculty of Agriculture, Kochi University, Monobe–Otsu 200, Nankoku, Kochi 783–8502, Japan (Received July 23, 2007) Summary A determination method for individual natural vitamin B6 compounds was developed. The vitamin B6 compounds were specifically converted into 4-pyridoxolactone (PAL), a highly fluorescent compound, through a combination of enzymatic reactions and HCl-hydrolysis. PAL was then determined by HPLC. Pyridoxal was completely oxidized to PAL with pyridoxal 4-dehydrogenase (PLDH). Pyridoxine and pyridoxamine were totally converted into PAL through a coupling reaction involving pyridoxine 4-oxidase and PLDH, and one involving pyridoxamine-pyruvate aminotransferase and PLDH, respectively. The 5¢- phosphate forms and pyridoxine-␤-glucoside were hydrolyzed with HCl, and then deter- mined as their free forms. Pyridoxine 5¢-phosphate and pyridoxine-␤-glucoside were not separately determined here. Three food samples were analyzed by this method. Key Words vitamin B6, 4-pyridoxolactone, pyridoxal 4-dehydrogenase, pyridoxine 4-oxi- dase, pyridoxamine-pyruvate aminotransferase There are six natural forms of vitamin B6: pyridoxine, hydrolyzed with HCl for the microbioassay, and pyridox- pyridoxal, pyridoxamine, pyridoxine 5¢-phosphate, pyri- ine-␤-glucoside in the samples is converted into pyri- doxal 5¢-phosphate, and pyridoxamine 5¢-phosphate doxine. The best method so far developed for individual (Fig. 1). In addition, plants contain pyridoxine-␤-gluco- determination may be cation-exchange HPLC with a side, a storage form of vitamin B6; it is not counted as fluorescence detection system (6), which is applicable to vitamin B6 because its nutritional availability as a form biological samples, such as serum, brain, liver, milk and of vitamin B6 in the human body is controversial (1). so on, from experimental animals. The second method The above six vitamin B6 compounds have identical applicable to samples containing specifically high nutritional functions. On the other hand, recent studies amounts of vitamin B6 compounds may be reversed- have shown that individual vitamin B6 compounds phase isocratic HPLC with a fluorescence detection sys- exhibit specific biochemical functions; pyridoxamine, in tem (7). However, these methods and other ones particular, can detoxify active carbonyl products and is recently reported (8–11), which are based on HPLC sep- undergoing a phase II clinical trial for the treatment of arations, suffer the disadvantage of difficulties for apply- complications of diabetes (2). Vitamin B6 compounds ing to analyses of foods, especially plants and beans, are strong quenchers of singlet oxygen (3), and pyri- because they contain only low amounts of individual doxal 5¢-phosphate and pyridoxamine 5¢-phosphate, vitamin B6 compounds together with much higher coenzyme forms of vitamin B6, show stronger protec- amounts of fluorescent components which are eluted as tion of yeast cells from oxidative death than vitamin C, interfering peaks on HPLC. a representative antioxidative vitamin (4). Thus, the To determine individual vitamin B6 compounds in development of a method for the determination of indi- such foods, both the specificity and sensitivity of the vidual vitamin B6 compounds plus pyridoxine-␤-gluco- methods should be improved. One promising way to side in foods is required to estimate their functionality. increase the specificity is to harness the functions of Although many methods have been developed for the enzymes. Recently, we cloned and expressed several determination of individual vitamin B6 compounds, no enzymes involved in the degradation pathway for pyri- method can be satisfactorily applied to the analysis of doxine (12–14). Three of them, when they are used food samples. Thus, a microbioassay involving a bud- alone or in combination, can specifically convert the ding yeast, Saccharomyces cerevisiae (former name, S. free (unphosphorylated) forms of vitamin B6 into a uvarum), which was developed about 40 y ago (5), has highly fluorescent vitamin B6-derivative, 4-pyridoxolac- been used to determine the total content of vitamin B6 tone (15), which can be measured in pmol amounts by in food samples; in the figure, the content of pyridoxine- HPLC. ␤-glucoside is included because food samples are pre- Here, we have developed a new method (designated as the Enzyme-HPLC method) for the determination of *To whom correspondence should be addressed. individual vitamin B6 compounds. We previously devel- E-mail: [email protected] oped an enzymatic fluorometric assay for pyridoxal 18 Individual Assaying of Vitamin B6 19 Fig. 1. Strategy for the enzyme-HPLC assay for natural vitamin B6 compounds and pyridoxine-␤-glucoside. All vitamin B6 compounds were enzymatically converted into 4-pyridoxolactone, a highly fluorescent compound, which was then deter- mined by HPLC. with high specificity and sensitivity (16), and the new were hydrolyzed at 121˚C with HCl to convert them method is an advanced version for individually deter- into the corresponding free forms according to the mining natural vitamin B6 compounds. AOAC official method for the determination of vitamin B in foods samples (AOAC Official Method 961.15). MATERIALS AND METHODS 6 Then, they are converted into 4-pyridoxolactone as Strategy. The strategy for the Enzyme-HPLC described above. method for the determination of individual vitamin B6 Materials. Recombinant pyridoxal 4-dehydrogenase is shown in Fig. 1. Pyridoxine, pyridoxamine, and pyri- (14), pyridoxine 4-oxidase (12), and pyridoxamine- doxal are specifically converted into 4-pyridoxolactone pyruvate aminotransferase (13) were prepared as previ- by three enzymes involved in the degradation pathway ously reported. Pyridoxine 5¢-phosphate was prepared for vitamin B6, and then 4-pyridoxolactone is deter- from pyridoxal 5¢-phosphate through reduction with mined by reversed-phase isocratic HPLC. Pyridoxal is sodium borohydride. Pyridoxine and pyridoxal 5¢-phos- converted into 4-pyridoxolactone by pyridoxal 4-dehy- phate were purchased from Nacalai Tesque, Inc. (Kyoto, drogenase, which catalyzes the irreversible oxidation of Japan), and pyridoxamine and pyridoxal from Sigma pyridoxine to 4-pyridoxolactone (14). Pyridoxine is con- Chemical Co. (St. Louis, MO). Pyridoxamine 5¢-phos- verted into 4-pyridoxolactone through a coupling reac- phate was a gift from Daiichi Fine Chemicals Co. tion involving pyridoxine 4-oxidase (12) and pyridoxal (Takaoka, Japan). Pyridoxine-␤-glucoside was a gift 4-dehydrogenase: both reactions are irreversible. Pyri- from Dr. H. Tsuge (Chubu University, Aichi, Japan) and doxamine is converted into 4-pyridoxolactone through Dr. K. Tadera (Kagoshima University, Kagoshima, a coupling reaction involving pyridoxamine-pyruvate Japan). aminotransferase (13) and pyridoxal 4-dehydrogenase. Sample preparation. All food samples were dried Although the transamination reaction is reversible, the according to the AOAC official method, and then coupled reaction becomes irreversible because pyri- ground with a Mini Blender (Osaka Chemicals, Osaka, doxal 4-dehyrogenase catalyzes no reverse reaction. Japan). For the determination of pyridoxine, pyridoxal Thus, pyridoxamine would be totally converted into 4- and pyridoxamine, the dried powder (0.05 g) of each pyridoxolactone. vegetable, legume, and grain was suspended in 10 mL Three phosphate forms and pyridoxine-␤-glucoside of cold 0.88 M HCl, and then kept for 5 h on ice. For 20 NISHIMURA S et al. determination of their 5¢-phosphate forms and pyridox- min B6 compounds into 4-pyridoxolactone. ine-␤-glucoside, the suspensions were autoclaved at Reaction conditions for the enzymatic conversion: 121˚C for 5 h. The cold extracts and autoclaved solu- Pyridoxal was converted into 4-pyridoxolactone at 30˚C tions were neutralized with 8.8 M NaOH, and then the for 1 h in a 600 ␮L capped Eppendolf tube containing neutralized solutions were passed through a glass filter, 180 ␮L of a reaction mixture consisting of 20 mM GF/A (Whatman, Maidstone, England). Generally, 50 sodium phosphate buffer (pH 8.0), 1 mM NAD, 1 mU ␮L of a filtrate was used for the next enzymatic reaction. pyridoxal 4-dehydrogenase, and 0.5–200 pmol of pyri- For other vegetable foods such as pepper and garlic, doxal. The enzyme reaction was stopped by the addition 0.44 M HCl was used. Meat was treated with 0.055 M of 20 ␮L of 0.44 M HCl. The reaction mixture was fil- HCl. tered through a Dismic-13 cp (pore size of 0.2 ␮m, Internal standards (almost the same amount as that Advantec, Tokyo, Japan), and then 100 ␮L of the filtrate found in the food samples for each vitamin B6 com- was injected into the HPLC column. To convert pyridox- pound) were added to the ground food samples just amine and pyridoxine into 4-pyridoxolctone, 4 mM before making the suspension. pyruvate and 1 mU pyridoxamine-pyruvate ami- Analytical method notransferase, and 5 ␮M FAD and pyridoxine 4-oxidase, HPLC system: 4-Pyridoxolactone was measured by a respectively, were added to the reaction mixture. reversed-phase isocratic HPLC method. It was separated Calculation of contents: Because food samples con- with a Jasco HPLC system (JASCO, Tokyo, Japan) tain all forms of vitamin B6, the pyridoxal content in equipped with a PU-2080 pump, an AS-2055 autosam- them was
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