Binding position of tolbutamide to human serum albumin

The interaction between drugs (tolbutamide (1), 1-butyl-3-(methylsulfonyl)urea (2)) and human serum albumin (3) was investigated by equilibrium dialysis and NMR spectroscopy. The binding of 1 and 2 to 3 was concluded to be hydrophobic and hydrophilic, respectively, on the basis of the dependence of the binding constants on temperature, ionic strength, and chain length of fatty acid added. In H-1-NMR spectra of 1, there were no significant shifts with change in concentration or addition of 3, The spin-lattice relaxation time (T-1) and spin-spin relaxation rate (1/T-2) of the respective protons of 1 were independent of concentration, but depended on the concentration of 3 added. The binding position was determined from the ratio of 1/T-2 of 1 bound to 3 and free 1. 1 and 2 were found to bind to 3 through the tolyl group and sulfonylurea group, respectively. The binding property of 1 was considered to be governed by the competition between the hydrophobic effect of the tolyl group and the hydrophilic effect of the sulfonylurea group in the molecule.