Membrane transport ofp-nitrophenyl-α-galactoside by the melibiose carrier ofEscherichia coli

p-Nitrophenyl-α-galactoside (α-pNPG) was found to be a substrate for the melibiose transport system ofEscherichia coli. This sugar enters induced cells via the carrier and is split by α-galactosidase to galactose andp-nitrophenol. In mutant cells lacking the α-galactosidase [3H]-α-pNPG accumulated to concentrations 15 times higher than the external medium. The transport of α-pNPG is inhibited by both Na+ and Li+. Na+ (10mm) reduced the Km for α-pNPG from 0.45 to 0.18mm and reduced theVmax from 6.7 nmoles/min/mg dry wt to a value of 3.0.