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Preparation and Specificities of Antisera to the Amino-terminal Sequence of the Carcinoembryonic Antigen¹

Departments of Biochemistry, Otolaryngology, Maxillofacial Surgery [D. P., K. R., B. A.], and Physiology [G. F.], Northwestern University Medical and Dental Schools, Chicago, Illinois 60611, and Laboratory of Immunology, Abbott Laboratories, North Chicago, Illinois 60064 [J. T. T., J. S.]

A tetracosapeptide (peptide-24) corresponding to the aminoterminal sequence of the carcinoembryonic antigen (CEA) was synthesized and characterized. Antisera were produced to the peptide-24, and a radioimmunoassay was developed utilizing peptide-24 with a tyrosine residue on the amino-terminal end (Tyr-peptide-24). Inhibitions of anti-peptide-24-¹²⁵I-Tyr-peptide-24 complex formation were done with several preparations of CEA and the normal cross-reacting antigen. The extent of cross-reactivities was low, one CEA preparation requiring a 250-fold molar quantity greater than peptide-24 to obtain the same degree of inhibition. Attempts to degrade the CEAs and normal cross-reacting antigens in order to possibly expose the amino-terminal ends for reactivity with antibody did not result in any great increase in inhibitory capacity. It was concluded that either the conformations of the antigenic determinant(s) of the peptide-24 and of the amino-terminal end of CEA were sufficiently different to result in little cross-reactivity or that the amino-terminal end of CEA and/or normal cross-reacting antigen are blocked for reactivity with antibody by other portions of the molecule.

¹ These studies were supported in part by Grant CA15145 awarded by the National Cancer Institute to the Northwestern University Cancer Center.

² Recipient of a Research Career Development Award, National Institute of Arthritis, Metabolism, and Digestive Diseases, AM-70787. To whom requests for reprints should be addressed.

Received 7/24/80. Accepted 3/ 5/81.

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