protein binding studies of furosemide and penbutolol serum protein binding of furosemide and penbutolol the active principles of betasemid hoe 9358 was studied by equilibrium dialysis membranes from commercial dialysis tubes were used within commercially available cells serum drug and portion unbound in buffer were determined by quantitative thinlayer chromatography in the range of 120 micrograms drugml serum 96 03 of furosemide vs 88 4 of penbutolol were bound to proteins the same results were obtained when the two substances simultaneously interacted with proteins thus specific protein binding sites for both compounds were demonstrated for both drugs scatchard plots revealed two classes of specific binding sites with statistical mean valuesprotein of 04 and 5 in the case of furosemide and 004 and 03 in the case of penbutolol binding energies were 276 and 196 kjmol furosemide vs 315 and 239 kjmol penbutolol further serum protein binding of furosemide was studied by ultrafiltration a micropartition system mps1 was used results were the same as those from equilibrium dialysis