Interaction between human serum albumin and α₁-acid glycoprotein in the binding of lidocaine to purified protein fractions and sera

The authors have determined the binding parameters for [¹⁴C] lidocaine to several preparations of purified human serum α₁-acid glycoprotein (AAG) and human serum albumin (HSA). The authors find that the average K(d) for the three different purified AAG preparations was 15.2±0.5μM, with an average of 0.567±0.009 binding sites (N) per AAG molecule, when the concentration of AAG was determined by a standard colorimetric protein assay. The number of binding sites per AAG increased to 0.99±0.06 when the concentration of AAG was determined immunologically. On the other hand, delipidized AAG had a K(d) of 28.0 μM, with an N of 1.72 based on the immunological assay. Various preparations of HSA had K(d)/N values ranging from 4.3 to 17.3 mM. The number of binding sites per HSA molecule could not be determined because the maximum concentration of lidocaine that can be used is 10 mM, which is only approximately equal to the K(d). The K(d) to HSA was so low and the capacity so high that, at therapeutic concentrations, the binding was in the linear range. Solutions containing both purified AAG and HSA had K(d) values of 30.2±1.8 μM, with 2.,01±0.04 binding sites per AAG molecule. With HSA and delipidized AAG, the K(d) was 60 μM, with 3.16 sites per delipidized AAG molecule. This increase in the binding parameters appeared to be due to a direct interaction between the two proteins. The binding of lidocaine to sera from five normal volunteers indicated that the apparent K(d) was 18.1±0.7 μM with N = 1.95±0.08 based on the immunological assay. These results would suggest that AAG and HSA interact in the serum as they do in the studies of the purified proteins. Finally, these data indicate that the K(d) of lidocaine in human serum is in the range of the total therapeutic serum concentrations so that the drug will show concentration-dependent binding.