Abstract
An overshoot phenomenon in ^3H-pyridoxine (^3H -PIN) uptake was observed with cells of a vitamin B_6 auxotroph (KG980) of Escherichia coli K12,as well as its wild strain. The overshoot was clearly detected with higher ^3H-PIN concentrations, e.g., 10μM in the medium and was most remarkable with the cells harvested in log phase of growth. This phenomenon appeared specific for PIN since little or no overshoot was observed with ^3H-pyridoxal or ^3H-pyridoxamine. ^3H-Pyridoxal phosphate and ^3H-pyridoxamine phosphate were the two major forms of intracellular ^3H-compounds at the time of maximal accumulation and were also the major forms of ^3H-compounds released subsequently into medium. The results have indicated that the phosphate forms of vitamin B_6 can cross E. coli cell membranes from inside to outside.
