A Comparison of Methods for the Isolation of Carcinoembryonic Antigen

Carcinoembryonic antigen (CEA) was isolated from a human tumor with 0.02 m sodium phosphate containing 0.14 m NaCl (pH 7.0) (saline) and further studied after treatment with perchloric acid or 8 m urea. Preparations of CEA obtained from saline homogenates and both methods of treatment were characterized by isoelectric focusing and gel filtration. Perchloric acid treatment resulted in an approximate 10-fold decrease in protein and a significant loss of CEA as compared to the saline- and urea-treated material. Isoelectric focusing revealed that the resultant CEA subpopulations were dependent upon the method of isolation. Urea- and saline-treated material showed complex isoelectric patterns that were quantitatively dissimilar. Perchloric acid-treated material showed a comparatively simple isoelectric pattern that was not significantly affected by electrofocusing in the presence of urea. Gel filtration on ACA 34 of the CEA obtained from each method of isolation resulted in two peaks of activity. The first peak corresponded to the void volume of the column, and the second peak coeluted with commercially available purified 125I-labeled CEA. Centrifugation of the peaks obtained resulted in a significantly greater loss of CEA from the void peak of each isolation procedure. The amount of CEA lost from the void peaks following centrifugation differed with each method of isolation and suggested the presence of aggregates.