Large immunologic species of human chorionic gonadotropin in placental extracts.

Abstract

Gel filtration of extracts of human chorionic tissues cultivated in vitro and of culture media has been performed. Heterogeneous immunoreactive forms of hCG and its subunits in tissue extracts were apparent. A void-volume immunoreactive peak of hCG was identified in extracts of chorionic tissues cultivated for a very short period. The immunoreactive void-volume material, which did not readily convert to the authentic form of hCG either spontaneously in refiltration or in 5 M guanidine, could not be distinguished immunologically from purified hCG and yielded an hCG-like component in trypsinization.On gel filtration of chorionic tissue extracts, radioreceptor activity for hCG utilizing membrance fraction of pseudopregnant rat ovary always demonstrated a single peak in the region of 125I-hCG used as column marker, indicating high receptor activity in the authentic form of hCG and low in the large immunologic species of hCG. Heterogeneity in the molecular size of bicactive hCG was not found with radioreceptor assay.On the other hand, gel filtration profiles of chorionic tissue extracts and of culture media showed that the large immunologic species of hCG is a predominant form of the hormone in the chorionic tissue cultivated for a short period and the authentic form of hCG is a predominant form in the media. These data suggest that immunoreactive “big hCG” synthesized initially in placenta may be converted to the authentic form of hCG by trypsin-like enzyme and that the authentic form of hCG may be readily secreted into the media. These results support the proposal that hCG is also synthesized as prohormone in placenta. However, the biosynthetic process of hCG subunits remains yet to be resolved.