Electrophoresis of reacting systems involving sulfhydryl oxidation of proteins

Numerical solution of the relevant continuity equations has been used to examine the possible effects of intramolecular sulfhydryl oxidation on the electrophoresis of proteins. Simulations of moving boundary electrophoresis, based on variants of a previous model [J.R. Cann, N.H. Fink, and D.J. Winzor (1983) Arch. Biochem. Biophys. 221, 57-63], show that the Schlieren patterns for the ascending and descending limbs are likely to exhibit pronounced nonenantiography. Whereas the pattern for one limb may comprise essentially a single peak, that for the conjugate side can exhibit bimodality, the nature of which is time dependent. Bimodality of the Schlieren pattern can develop in either the ascending or descending limb of the electrophoresis cell, depending basically upon the number of sulfhydryl groups available for oxidation, and on the relative magnitudes of the rate constants describing the oxidation and the isomerization of the oxidized protein species. Whether the faster-moving or slower-moving peak grows with time is shown to depend upon the magnitude of the electrophoretic mobility of the resultant isomer in relation to that of the oxidized protein species. Schlieren patterns for fish muscle creatine kinase and rabbit muscle aldolase are then used to support the relevance of these predictions to moving boundary electrophoresis of proteins undergoing intramolecular sulfhydryl oxidation. Finally, numerical simulation of the zonal electrophoretic behavior of such systems serves to illustrate that bimodal patterns may also obtain, thereby giving a false impression of inherent protein heterogeneity. Emphasis is therefore placed on the importance of maintaining an adequate concentration of reducing agent throughout the medium in which the protein migrates, a potential problem in polyacrylamide gel electrophoresis at neutral pH.