Pyridoxine-5′-β-D-glucoside Exhibits Incomplete Bioavailability as a Source of Vitamin B-6 and Partially Inhibits the Utilization of Co-Ingested Pyridoxine in Humans ,

Abstract

This research was conducted to investigate 1) the bioavailability of pyridoxine-5'-beta-D-glucoside (PN-glucoside) relative to that of pyridoxine (PN) in human subjects, and 2) the competitive effect of PN-glucoside on the metabolism of co-ingested PN. To evaluate PN-glucoside bioavailability, the subjects were administered a single oral dose of either deuterium-labeled ([2H2]) PN (Trial 1) or [2H2] PN-glucoside (Trial 2), and the urinary excretion rates of labeled 4-pyridoxic acid (4PA) were measured. The [2H2]4PA derived from [2H2] PN or [2H2]PN-glucoside was excreted mainly in the first 8 h after the dose. Excretion of [2H2]4PA during the 48-h postdose period indicated that the bioavailability of PN-glucoside was approximately 50% relative to PN, which is consistent with our previous report of 58% bioavailability determined using a different protocol and fewer subjects. To assess the effects of PN-glucoside on PN utilization, the subjects were administered different ratios of nonlabeled PN-glucoside with [2H2]PN in Trials 3 and 4. Comparing Trial 1 with Trials 3 and 4, the quantity of nonlabeled PN-glucoside, as a fraction of total vitamin B-6 administered, ranged from 0 to 40% (on the basis of pyridoxine equivalents), with a constant dose of [2H2]PN in each. In these trials, the rate but not the total extent of the excretion of [2H2]4PA derived from [2H2]PN was inversely related to the proportion of co-ingested nonlabeled PN-glucoside. Thus, antagonistic effects of PN-glucoside on PN metabolism do occur in humans, although the effect is less pronounced than that seen previously in rats. Such interactive effects must be considered in evaluating the net bioavailability of dietary forms of vitamin B-6.

Enzymatic Hydrolysis of Pyridoxine-5′-β-d-glucoside Is Catalyzed by Intestinal Lactase-Phlorizin Hydrolase

“…Dietary PNG exhibits only 50 -60% of the bioavailability of free PN as a source of vitamin B 6 for humans (3,4), which is due to incomplete hydrolysis of the glycosidic linkage of PNG. The results reported in the present study suggest that the digestion of PNG may commence at the intestinal brush border where LPH can catalyze the hydrolysis of PNG to release free PN, which is then passively absorbed.…”

“…1 PNG, found predominantly in foods of plant origin, provides 15% of total vitamin B 6 in a mixed diet, depending on food selection (1,2). PNG exhibits ϳ50% bioavailability in humans (3,4) and 25-30% in rodents (5)(6)(7)(8) in comparison to pyridoxine, the metabolically usable form of vitamin B 6 . The rate-limiting step in the utilization of PNG is not its intestinal absorption but rather the enzymatic hydrolysis of the ␤-glucosidic linkage to release glucose and pyridoxine in the intestine (6 -8).…”

HPLC analysis of vitamin B6 in foods

“…HPLC analysis of vitamin B 6 in foods β-glycosidic bond is reported to be stable in nonmineral acid solutions, such as in perchloric acid solution Bitsch 1993, ToukairinOda et al 1989), in trichloroacetic acid solution Ink 1987, van Schoonhoven et al 1994) and in metaphophoric acid solution (Sampson et al 1995), or in neutral buffer solutions (Kabir et al 1983a) but it is hydrolyzed in mineral acids especially when acid extraction is combined with a thermal process like autoclaving (Kawai et al 1971a). Preparation of α-glycosylated pyridoxines (Suzuki et al1997) and nonlabeled, deuterated as well as tritiated β-glucosidic forms (Gregory and Nakano 1997) has been recently reported.…”

Cytosolic Pyridoxine-β-d-Glucoside Hydrolase from Porcine Jejunal Mucosa

“…It will be of interest to determine the distribution of pyridoxine-␤-D-glucoside hydrolase among tissues as well as to determine the cross-reactivity among the various mammalian cytosolic and lysosomal ␤-glucosidases. In addition, immunochemical analysis will be essential in clarifying the mechanism by which vitamin B6 deficiency causes increased activity of both intestinal broad specificity ␤-glucosidase and pyridoxine-␤-D-glucoside hydrolase (7).…”

“…Because of the prevalence of PNG in plantderived foods, its bioavailability as a source of available vitamin B6 is a matter of nutritional concern. The bioavailability of this glycosylated form of vitamin B6, relative to pyridoxine (PN), is ϳ25% in rats (4,5) and ϳ50% for humans (6,7), as estimated from urinary excretion of 4-pyridoxic acid after oral administration of isotopically labeled PNG. The rate-limiting phase of PNG utilization in vitamin B6 metabolism is the hydrolysis of the ␤-glycosidic bond in both rats and humans.…”