AFlCIiIVES
OF
Kinetics
BIOCHEMISTRY
AND
BIOPHYSICS
282-289
184,
(1977)
of the Binding of Bilirubin to Human Serum Albumin by Stopped-Flow Technique THYGE
Institute
of Medical
FLERCH’
Biochemistry,
JBRGEN
AND
University Received
Studied
JACOBSEN
of Aarhus,
DK-8000
Aarhus
C, Denmark
June 20, 1977
The binding of bilirubin to human serum albumin has been studied in stopped-flow measurements of the absorbance at 472 nm. The binding kinetics at 5.5% and pH 8.8 is described by a second-order rate constant of 1.6 x 10’ M-’ s-l, and four first-order rate constants in five consecutive steps. The absorption spectrum of the short-lived complex initially formed has been determined in the range from 420 to 490 nm. MATERIALS
, HSA* and BSA both bind one molecule of bilirubin with high affinity, the binding constant being about lOa M-’ (l-3). Chen (4) has studied the kinetic course of the association of bilirubin with albumins from several species, including man, by use of the stop-flow technique. He showed that the binding takes place in several steps, but was unable to follow any primary association steps, though he proved their existence. In a continuous-flow system we have previously investigated the binding of bilirubin to albumin at 36°C and pH 7.4, using the absorbance at 474 nm (5). An estimate of the second-order rate constant was obtained for BSA but not for HSA. In the present work the binding of bilirubin to HSA is studied with stopped-flow measurements of the absorbance at 472 nm. The second-order rate constant for the primary binding is determined at 5.5”C and pH 8.8. The changes in absorption observed after the rapid binding are described by four successive first-order steps, the rate constants of which are estimated. Hence, a total model for the kinetic course of the binding of bilirubin to HSA is proposed. 1 Present address: Department of Clinical Chemistry, Aalborg Sygehus, DK-9000 Aalborg, Denmark. * Abbreviations used: HSA, human serum alhumin; BSA, bovine serum albumin.
AND
METHODS
Crystalline bilirubin was obtained from Sigma Chemical Co., and lyophilized HSA was from KABI, Sweden. The albumin was defatted according to Chen (6). Fatty acid concentration was measured by the method of Dole (7). The experiments were carried out in an AmincoMorrow stopped-flow apparatus, and the changes in absorbance were monitored with a modified Beckman DU spectrophotometer. The output from the photometer was recorded on a Tectronix storage oscilloscope and, for the slowest step, a Philips x-y recorder equipped with a time base was used. Constant temperature in the mixing chamber was maintained by circulating water from a thermostated bath. The temperature was measured in the effluent tube. The observation cell was flushed with dry nitrogen to prevent water condensation on the windows. The oscilloscope display was photographed with a Polaroid camera, and the picture could be used directly for the analysis. The stopped-flow apparatus was tested according to the manual, and the results were in good agreement with the given technical data. A dead time in the range of 2.5-4 ms was thereby assumed. In the experiments, equal volumes of solutions of HSA and bilirubin were mixed. Both compounds were dissolved in 0.01 M Tris buffer, pH 8.8, at 5.5%. The various concentrations of HSA and bilirubin given in this article all refer to final concentrations in the mixing chamber. All solutions were prepared immediately before use; the bilirubin solutions were protected from light. RESULTS
The absorption
spectra of bilirubin
and
282 Copyright All rights
8 1977 by Academic Press, Inc. of reproduction in any form reserved.
ISSN
0003-9361
BINDING
OF BILIRUBIN
TO HUMAN
of the final bilirubin-albumin complex in the Tris buffer, pH 8.8, are shown in Fig. 1. The molar absorptivities at 472 nm of the two compounds (eb and EJ can be taken directly from the figure. From this it is obvious that the ultimate result of binding of bilirubin to HSA is an increase in absorbance at 472 nm. However, the oscilloscope display (Figs. 2 and 3) shows a primary increase in absorbance followed by a decrease, another increase, and a decrease to the final level, which is reached in a few seconds. The first step is very fast and can be studied only at relatively low temperatures (
OF
Kinetics
BIOCHEMISTRY
AND
BIOPHYSICS
282-289
184,
(1977)
of the Binding of Bilirubin to Human Serum Albumin by Stopped-Flow Technique THYGE
Institute
of Medical
FLERCH’
Biochemistry,
JBRGEN
AND
University Received
Studied
JACOBSEN
of Aarhus,
DK-8000
Aarhus
C, Denmark
June 20, 1977
The binding of bilirubin to human serum albumin has been studied in stopped-flow measurements of the absorbance at 472 nm. The binding kinetics at 5.5% and pH 8.8 is described by a second-order rate constant of 1.6 x 10’ M-’ s-l, and four first-order rate constants in five consecutive steps. The absorption spectrum of the short-lived complex initially formed has been determined in the range from 420 to 490 nm. MATERIALS
, HSA* and BSA both bind one molecule of bilirubin with high affinity, the binding constant being about lOa M-’ (l-3). Chen (4) has studied the kinetic course of the association of bilirubin with albumins from several species, including man, by use of the stop-flow technique. He showed that the binding takes place in several steps, but was unable to follow any primary association steps, though he proved their existence. In a continuous-flow system we have previously investigated the binding of bilirubin to albumin at 36°C and pH 7.4, using the absorbance at 474 nm (5). An estimate of the second-order rate constant was obtained for BSA but not for HSA. In the present work the binding of bilirubin to HSA is studied with stopped-flow measurements of the absorbance at 472 nm. The second-order rate constant for the primary binding is determined at 5.5”C and pH 8.8. The changes in absorption observed after the rapid binding are described by four successive first-order steps, the rate constants of which are estimated. Hence, a total model for the kinetic course of the binding of bilirubin to HSA is proposed. 1 Present address: Department of Clinical Chemistry, Aalborg Sygehus, DK-9000 Aalborg, Denmark. * Abbreviations used: HSA, human serum alhumin; BSA, bovine serum albumin.
AND
METHODS
Crystalline bilirubin was obtained from Sigma Chemical Co., and lyophilized HSA was from KABI, Sweden. The albumin was defatted according to Chen (6). Fatty acid concentration was measured by the method of Dole (7). The experiments were carried out in an AmincoMorrow stopped-flow apparatus, and the changes in absorbance were monitored with a modified Beckman DU spectrophotometer. The output from the photometer was recorded on a Tectronix storage oscilloscope and, for the slowest step, a Philips x-y recorder equipped with a time base was used. Constant temperature in the mixing chamber was maintained by circulating water from a thermostated bath. The temperature was measured in the effluent tube. The observation cell was flushed with dry nitrogen to prevent water condensation on the windows. The oscilloscope display was photographed with a Polaroid camera, and the picture could be used directly for the analysis. The stopped-flow apparatus was tested according to the manual, and the results were in good agreement with the given technical data. A dead time in the range of 2.5-4 ms was thereby assumed. In the experiments, equal volumes of solutions of HSA and bilirubin were mixed. Both compounds were dissolved in 0.01 M Tris buffer, pH 8.8, at 5.5%. The various concentrations of HSA and bilirubin given in this article all refer to final concentrations in the mixing chamber. All solutions were prepared immediately before use; the bilirubin solutions were protected from light. RESULTS
The absorption
spectra of bilirubin
and
282 Copyright All rights
8 1977 by Academic Press, Inc. of reproduction in any form reserved.
ISSN
0003-9361
BINDING
OF BILIRUBIN
TO HUMAN
of the final bilirubin-albumin complex in the Tris buffer, pH 8.8, are shown in Fig. 1. The molar absorptivities at 472 nm of the two compounds (eb and EJ can be taken directly from the figure. From this it is obvious that the ultimate result of binding of bilirubin to HSA is an increase in absorbance at 472 nm. However, the oscilloscope display (Figs. 2 and 3) shows a primary increase in absorbance followed by a decrease, another increase, and a decrease to the final level, which is reached in a few seconds. The first step is very fast and can be studied only at relatively low temperatures (
![Photoactivated covalent binding of [3H]bilirubin to human serum albumin.](/assets/img/hold.png)
