Fragmentation of the beta-subunit of human chorionic gonadotropin produced by choriocarcinoma

Abstract

When human chorionic gonadotropin (hCG) was subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions with dithiothreitol (DTT), a smaller weight material (CTP'), in addition to the beta-subunit, could be detected by Western blot analysis using antiserum for hCG beta-carboxy-terminal peptide (CTP). The CTP' band was much more apparent with urinary hCG from a patient with choriocarcinoma than with that from normal pregnant women. Second-dimensional electrophoresis of the choriocarcinoma hCG (c-hCG) after reduction with DTT indicated that the CTP', Mr 25,000, was released from the beta-subunit. The carbohydrate structure of the CTP' was analyzed by affinity with lectin-peroxidase on a nitrocellulose membrane. The CTP' did not interact with Concanavalin A, but exhibited strong interaction with both RCA120 and Arachis hypogaea after removal of sialic acid, indicating that it was released as a fragment containing an O-linked sugar chain as was found in the hCG beta carboxy-terminal portion. Western blot analysis using the antisera for hCG alpha, hCG beta, and hCG beta-CTP showed that the CTP' contains not only the carboxy-terminal portion but also a part of the internal (core) portion of the beta-subunit molecule. This dissociation of the c-hCG beta was further supported by the presence of a faster moving component (FMC) which may correspond to the NH2-terminal side counterpart. The desialylated FMC could be detected by Concanavalin A and RCA120 but not by Arachis hypogaea, indicating that it contains N-linked rather than O-linked sugar chains. The FMC does not contain any of the epitopes for the antisera examined in Western blot. These results indicate that the beta-subunit of the choriocarcinoma urine hCG has an unusual site which is dissociated into two components of Mr 25,000 (CTP') and Mr 18,000 (FMC) by DTT reduction.