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Research was conducted to evaluate the ability of a broad-specificity beta-glucosidase in mammalian tissues to catalyze the hydrolytic release of free pyridoxine from pyridoxine-5'-beta-D-glucoside, a naturally occurring form of vitamin B6 in plant-derived foods. Activity was detected in liver and intestinal mucosa using tritiated pyridoxine glucoside as a substrate. In the rat and guinea pig, enzyme activity was greater in intestine than in liver or kidney while even greater activity was detected in human intestinal tissue. Reaction rates were, however, low in all tissues. Hydrolysis of the synthetic substrate 4-methylumbelliferyl-beta-D-glucoside was also greatest in intestinal tissue. The characteristics of the enzymatic hydrolysis of pyridoxine glucoside to pyridoxine included: (i) most activity in the soluble tissue fraction, (ii) a pH optimum of approximately 6.0, and (iii) inhibition caused by the addition of sodium taurocholate. These characteristics are very similar to those of the broad-specificity beta-glucosidase in mammalian tissues with respect to the hydrolysis of a variety of naturally occurring and synthetic substrates. The apparent Km was greater than 2 mM for pyridoxine glucoside hydrolysis by intestinal preparations of each species, which is much greater than expected intestinal concentrations derived from dietary sources. In vivo studies have indicated that the intestine is involved in the metabolic utilization of dietary pyridoxine glucoside. The results observed here suggest that an alternate process, possibly involving intestinal microorganisms, may also be involved in the in vivo hydrolysis of pyridoxine glucoside.

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Evidence suggests that taurine may protect cellular membranes against oxidants (Gordon et al., Am. J. Pathol. 125: 585-600, 1986). The present study was conducted to determine if alveolar macrophages and type II cells (which are relatively resistant to oxidant injury) possess a specialized transport system for the accumulation of taurine. The results indicate that both cell types contain more taurine than plasma or whole lung. Taurine influx exhibited both carrier-mediated and simple diffusion components. Carrier-mediated uptake displayed saturation kinetics (Km = 26.3 and 22.5 microM, while Vmax = 33.2 and 4.9 pmol.10(6) cells-1.min-1 for macrophages and type II cells, respectively). Taurine uptake was dependent on extracellular sodium and inhibited by metabolic inhibitors or ouabain. Total taurine uptake by type II cells was lower than that of alveolar macrophages. However, type II cells exhibited a higher intercellular concentration of taurine (14 vs. 4 mM) because of a higher ratio of carrier-mediated uptake to leakage than with alveolar macrophages. It is possible that this specialized transport system for taurine uptake may lend these cells resistant to oxidant injury.