Binding of sulphamethazine to pig plasma proteins and albumin

The interaction of sulphamethazine (SMZ) with pig plasma proteins and albumin was studied by ultrafiltration and equilibrium dialysis. Binding to pig plasma proteins was monophasic (affinity approximately 9.0 mol/L x 10(3)) and the main binding protein was albumin. At 37 degrees C and pH 7.4, the affinity of SMZ for albumin was about 8.0 mol/L x 10(3) and the number of binding sites was estimated as 1.4. Increasing the temperature from 4 to 45 degrees C resulted in a seven-fold decrease in affinity, and increasing pH from 6.0 to 8.0 enhanced affinity for pig albumin ten-fold. The free energy of binding (-delta G) and enthalpy change (-delta H) were around 5.5 and 5.1 Kcal/mol, respectively. The total entropy change (delta S) was small and positive, around 2 cal/mol/degree K. Studies with the fluorescent probes warfarin and dansylsarcosine, suggest that these bind to separate sites on porcine albumin. SMZ displaced both probes and inhibited the deacetylation of p-nitrophenyl acetate by pig albumin. We conclude that: (1) binding of SMZ to pig plasma proteins and albumin is weak; (2) the interaction with albumin is exothermic and enthalpy driven, and (3) pig albumin, like other mammalian albumins, appears to possess discrete binding sites for warfarin and dansylsarcosine. SMZ interacts with both these loci.