Abstract
It has been observed that pyridoxine and pyridoxal added to the medium are uptaken into the cells of Escherichia coli and elevate the intracellular content of pyridoxal phosphate. In order to investigate the transport mechanism of pyridoxine into the cells, experiments were made of ^3H-pyridoxine transport by a mutant (KG 980) of E. coli, which requires a high concentration of vitamin B_6 for growth, isolated from E. coli K12 by nitrosoguanidine treatment. Uptake of ^3H-pyridoxine by the cells was found to be an energy-, temperature- and pH-dependent process. It was specifically inhibited by vitamin B_6 analogues. The enzymes, which are capable of conversion of pyridoxine to pyridoxal phosphate, were prepared from the sonic extracts of the mutants and parents. They were found in the soluble fraction of the sonic extracts, but in the membrane fraction in contrast with thiamine kinase from E. coli. From the investigation of enzyme activities, the pathway of pyridoxine to pyridoxal phosphate was assumed to be predominant in pyridoxine→pyridoxine phosphate→pyridoxal phosphate. The specific inhibition by vitamin B_6 analogues, especially 4-deoxypyridoxine seemed to be limited to the step of permeation, since they were not implicated in the conversion of pyridoxine to pyridoxal phoshate. This result suggests an existence of a "carrier protein" specific for pyridoxine transport through the cell membranes.
