Properties of a Na⁺/galactose(glucose) symport system in Vibrio parahaemolyticus

We have investigated galactose transport in a mutant strain of Vibrio parahaemolyticus that lacks a glucose-PTS (phosphoenolpyruvate:carbohydrate phosphotransferase system) and a trehalose-PTS. Cells of the V. parahaemolyticus actively transported D-galactose and Na⁺ greatly stimulated the transport. Maximum stimulation of D-galactose transport activity was observed at 10 mM NaCl, and Na⁺ could be replaced with Li⁺. Addition of galactose to the cell suspension under anaerobic conditions elicited Na⁺ uptake. Therefore, we conclude that this organism accomplishes galactose transport by a Na⁺/solute symport mechanism. Judging from inhibition results, D-galactose, D- glucose and to a lesser extent α-D-fucose are substrates of this transport system. The Na⁺/galactose symport system exhibited a high affinity for D-galactose (K(m): 40 μM) and showed a relatively lower affinity for D-glucose (K(m): 420 μM), but the maximum velocities for galactose and glucose transport were almost same (about 52 nmol/min per mg protein). The Na⁺/D-galactose symport system was induced by either D-galactose or α-D-fucose, and repressed by D-glucose.