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Moving boundary electrophoresis behavior and acid isomerization of human mercaptalbumin.

P. Clark,M. R. Rachinsky,J. F. Foster

Published 1962 in Journal of Biological Chemistry

ABSTRACT

The behavior of bovine plasma albumin in moving boundary electrophoresis has been studied in great detail, especially by Aoki and Foster (l-6) and by Cann (7-10). In general terms, it has been found that this protein yields a single moving boundary at pH values above the isoelectric point (approximately pH 4.5) but two, and in some cases three, boundaries between pH 3.5 and 4.5. These results have been interpreted by Aoki and Foster (l-6) in terms of a cooperative alteration in the protein conformation, which takes place near pH 4. Some independent evidence for this “isomerization” has been obtained from solubility studies (11) and, more recently, from studies of optical rotation and rotatory dispersion (12, 13). It has also been suggested that the well known anomaly in the acid titration curve of this protein can be explained, in the main, as due to this phenomenon (6). At the time of initiation of the experiments reported here, no evidence existed for the isomerization of plasma albumins of any species other than bovine. It was shown earlier by Tanford, Swanson, and Shore (14) that the titration curves of human and bovine alhumins are essentially identical. This would imply that if indeed the titration anomaly in bovine plasma albumin is due to the isomerization reaction, the same reaction must take place in the human protein. However, Jirgensons (15) suggested that this may not be the case. We considered it of some importance, therefore, to examine the electrophoretic behavior of the human protein. Furthermore, since the plasma albumins are patently heterogeneous, in that some of the molecules contain free sulfhydryl groups and others do not, we were led to utilize the mercaptalbumin fraction of the protein by the thought that it might be more homogeneous. In the interim since this program was initiated, ample evidence has appeared that human albumin exhibits much the same type of isomerization behavior as bovine (12, 16-19). However, we know of no published electrophoretic studies that demonstrate the isomerization equilibrium in human mercaptalbumin nor, in fact, in the mercaptalbumin fraction of any albumin. A more important reason for presenting these results at this time is

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