Pyridoxine-5'-beta--glucoside exhibits incomplete bioavailability as a source of vitamin B-6 and partially inhibits the utilization of co-ingested pyridoxine in humans.

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.

Cytosolic pyridoxine-beta-D-glucoside hydrolase from porcine jejunal mucosa. Purification, properties, and comparison with broad specificity beta-glucosidase.

During studies of the nutritional utilization of pyridoxine 5'-beta-D-glucoside, a major form of vitamin B6 in plants, we detected two cytosolic beta-glucosidases in jejunal mucosa. As expected, one was broad specificity beta-glucosidase that hydrolyzed aryl beta-D-glycosides but not pyridoxine beta-D-glucoside. We also found a previously unknown enzyme, designated pyridoxine-beta-D-glucoside hydrolase, that efficiently hydrolyzed pyridoxine beta-D-glucoside. These were separated and purified as follows: broad specificity beta-glucosidase 1460-fold and pyridoxine-beta-D-glucoside hydrolase 36,500-fold. Purified pyridoxine-beta-D-glucoside hydrolase did not hydrolyze any of the aryl glycosides tested but did hydrolyze cellobiose and lactose. Pyridoxine-beta-D-glucoside hydrolase exhibited a pH optimum of 5.5 and apparent molecular mass of 130 kDa by SDS-polyacrylamide gel electrophoresis and 160 kDa by nondenaturing gel filtration, in contrast to 60 kDa for native and denatured broad specificity beta-glucosidase. Glucono-delta-lactone was a strong inhibitor of both enzymes. Ionic and nonionic detergents were inhibitory for each enzyme. Conduritol B epoxide, a potent inhibitor of lysosomal acid beta-glucosidase, inhibited pyridoxine-beta-D-glucoside hydrolase but not broad specificity beta-glucosidase, but both were inhibited by the mechanism-based inhibitor 2-deoxy-2-fluoro-beta-D-glucosyl fluoride. Our findings indicate major differences between these two cytosolic beta-glucosidases. Studies addressing the role of vitamin B6 nutrition in regulating the activity and its consequences regarding pyridoxine glucoside bioavailability are in progress.