Protein Binding of Phenprocoumon in the Absence and Presence of Furosemide

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Abstract
A modification of a fluorometric assay for phenprocoumon is described. The sensitivity of the method has been increased so that the lower limit is 5 ng/ml. By an ultrafiltration technique, phenprocoumon (PPC), in therapeutic concentrations, was bound to the extent of 99.9%, both in undiluted plasma and in Krebs–Henseleit (K–H) solutions containing 4 g human serum albumin (HSA)/100 ml. An increase in the pH from 6.05 to 8.02 in phosphate buffers containing 0.2 g HSA/100 ml yielded an increase in the apparent association constant for PPC binding to HSA from 0.562–106 1/mol to 1.195–1061/mol, while the number of PPC mol bound per mol albumin remain unchainged (1.05 and 1.03 respectively). A reduced binding of PPC in undiluted plasma containing increasing concentrations of furosemide have been observed. Scatchard plots based on binding studies in K–H solutions containing 0.2 g HSA/100 ml indicate a competitive nature of the albumin binding of PPC and furosemide.