Role of Pyridoxal Kinase in Vitamin B6 Uptake by Escherichia coli

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Escherichia coli KG980, a vitamin B6 auxotroph derived from wild strain K12, concentrated exogenous pyridoxal in an energydependent manner, and the effects of energy sources and inhibitors on pyridoxal uptake, compared with those on proline uptake indicated that the energy required was in the form of phosphate bonds and not of membrane potential. The vitamin taken up was primarily present as pyridoxal 5-phosphate and pyridoxamine 5-phosphate intracellularly, and energy depletion decreased the accumulation as the phosphorylated derivatives but not as unaltered pyridoxal itself. This finding suggested that the intracellular phosphorylation, which was known to require ATP, was essential for the concentrative uptake of the vitamin. The suggestion was confirmed by the following evidence. 1) Pyridoxal oxime inhibited pyridoxal uptake by decreasing the intracellular phosphorylation without affecting the entry of pyridoxal across the cell membrane. 2) A pyridoxalkinase deficient mutant (MN 1) derived from the strain KG980 showed a low ability to take up pyridoxal because of the failure to accumulate it effectively as phosphorylated derivatives. The carrier-mediated nature of pyridoxal uptake, previously suggested by saturation kinetics, was further supported by the present finding that 4-deoxypyridoxine inhibited pyridoxal uptake competitively, decreasing the intracellular appearance of unmetabolized pyridoxal. It is therefore most likely that pyridoxal enters the cells by facilitated diffusion and is accumulated by conversion to phosphorylated derivatives. Similar results on the uptake of pyridoxine and pyridoxamine are also presented.
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Role of Pyridoxal Kinase in Vitamin B6 Uptake by Escherichia coli

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