Identification and partial characterization of a lactogen from the midpregnant mouse conceptus

Previously, we demonstrated that measurement of serum lactogenic hormones during pregnancy in the mouse by the rabbit mammary gland radioreceptor assay (RRA) and specific RIAs for mouse PRL (mPRL) and mouse placental lactogen (mPL) showed a large RRA to RIA activity ratio during midpregnancy. The purpose of this study was to begin to characterize this discrepancy in midpregnancy PRL-like activity. The elution profile of serum from midpregnant mice on Sephadex G-100 showed a characteristic peak in RRA activity (elution volume/void volume = 1.5), distinct from either mPRL or mPL (both approximately elution volume/void volume = 2.0). This PRL-like activity was not recognized by antisera to either mPRL or mPL. The source of the midpregnancy lactogen was localized to the conceptus. The hormone was found to have an apparent molecular weight of 50,000 (based on Sephadex G-100 chromatography) and an electrophoretic mobility similar to mPRL (Rf = 0.5-0.6) and to be lactogenic in the in vitro mouse mammary gland explant bioassay. We have demonstrated the existence of a midpregnancy lactogen that is different from mPL and mPRL and is of conceptus origin.