Use of borate to control the 5'-position-selective microbial glucosylation of pyridoxine

All-enzymatic HPLC method for determination of individual and total contents of vitamin B(6) in foods

Background

There is a need for a reliable and accurate method for quantification of each of the seven individual vitamin B₆ compounds including pyridoxine-β-glucoside in foods.

Objective

To determine pyridoxal (PL), pyridoxamine (PM), pyridoxine (PN), pyridoxal 5′-phosphate (PLP), pyridoxamine 5′-phosphate (PMP), pyridoxine 5′-phosphate (PNP), and pyridoxine-β-glucoside (PNG) in foods.

Design

By specific enzymatic treatment, each of the seven vitamin B₆ compounds was all converted into 4-pyridoxolactone, which is a highly fluorescent compound. In total, seven separate, enzymatic steps were performed for each sample. Separation and quantification were performed with reversed-phase high performance liquid chromatography (HPLC) coupled with fluorescence detection. For each sample type the result was corrected for the recovery based on spiked samples. The method was applied for analyses of chicken liver, chicken white meat, egg yolk, egg white, dried anchovy, carrots, and garlic.

Results

The recovery varied from 14 to 114% in chicken liver, chicken white meat, egg yolk, egg white, dried anchovy, carrot, and garlic. Each food showed a characteristic distribution of the seven vitamin B₆ compounds. The PNG was only found in low amounts; that is, 17–29nmol vitamin B₆/g in the plant-derived foods, carrot and garlic. Only egg white showed a lower content, 3nmol/g. Overall the content in chicken liver, chicken white meat, and egg yolk had a total content of vitamin B₆ between 42 and 51nmol/g. Both PM and PMP were high in the chicken liver. In contrast, PL and PLP were high in the chicken white meat. The main vitamin B₆ in the egg yolk was PLP. The dried anchovy contained high amounts of PLP and PMP and a total content of 144nmol/g.

Conclusions

The enzymatic-based HPLC method was applied for the determination of seven vitamin B6 compounds in foods. Their distribution in the foods varied significantly.

Conversion shift of D-fructose to D-psicose for enzyme-catalyzed epimerization by addition of borate

The conversion yield of d-psicose from d-fructose by a d-psicose 3-epimerase from Agrobacterium tumefaciens increased with increasing molar ratios of borate to fructose, up to a ratio of 0.6. The formation of the psicose-borate complex was the result of the higher binding affinity of borate for psicose than for fructose. The formed psicose-borate complex did not participate in the conversion reaction, acting instead as if the product had been removed. Thus, more fructose was converted to psicose in order to restore the equilibrium. The maximum conversion yield of psicose with borate was about twofold that obtained without borate and occurred at a 0.6 molar ratio of borate to fructose. Above this ratio, the conversion yield decreased with increasing ratios, because the amount of fructose available decreased through the formation of the initial fructose-borate complex. The structures of the two sugar-borate complexes, determined by nuclear magnetic resonance spectroscopy, were alpha-d-psicofuranose cis-C-3,4 diol borate and beta-d-fructopyranose cis-C-4,5 diol borate.