A Growth Hormone Binding Factor in the Serum of Pregnant Mice

Using a radioreceptor assay (RRA) an apparent growth hormone-like activity (GH-LA) was first detected in the serum of Swiss Webster mice at mid-pregnancy with peak concentrations appearing one to two days before parturition. Hysterectomy at day 17 of pregnancy was followed by a transient increase in apparent GH-LA. In the RRA, displacement of human [125I]iodoGH from the receptor by serial dilutions of serum from pregnant mice paralleled that of purified human GH standards. After gel filtration of a sample of pregnant serum on a Sephadex G-100 column the apparent GHLA determined by RRA was found in fractions with a relative elution volume (Ve/Vo) of 1.2. Moreover, when a sample of serum from pregnant mice and human [125I]iodoGH were separated on a similar column, more than 75% of the labeled hormone also eluted with a relative elution volume (Ve/Vo) of 1.2 rather than 2.0 which was found when human [l25I]iodoGH alone or with serum from nonpregnant mice was fractionated on the same column. In these fractions more than 65% of the human [125I]iodoGH was precipitable by excess antiserum to hGH. Upon addition of increasing amounts of unlabelled hGH to the mixture ef human [l25I]iodoGH and serum from pregnant mice a progressive decrease in the amount of human [125I]iodoGH recovered in fractions with a Ve/Vo of 1.2 was found. Bovine GH, but not ovine prolactin (oPRL) caused a similar displacement. When a plasma sample was obtained from pregnant mice after injection of 10 μg of unlabelled hGH and was fractionated in a similar manner, immunoreactive hGH was recovered in eluates corresponding to a Ve/Vo of 1.2. After injection of hGH into nonpregnant mice, immunoreactive hGH was recovered in fractions eluting with a Ve/Vo of 2.0. Following the same procedure, both unlabelled oPRL and [125I]iodoPRL consistently eluted in fractions with Ve/Vo = 2.0. In vivo experiments showed that the half-time disappearance rate of human [125I]iodoGH was significantly longer in the pregnant than in the nonpregnant mouse. When fresh serum obtained from mice on day 18 of pregnancy was fractionated, the endogenous immunoreactive mouse GH was recovered in fractions which also contained the GHbinding factor. Immunoreactive mouse GH from pituitary extracts was shown to bind to this factor on mixing with serum from pregnant mice. Serum concentration of immunoreactive mouse GH appeared to increase during pregnancy. In vitro studies indicated that the growth hormone binding factor competitively inhibited the binding of human [125I]iodoGH to rabbit liver in the standard RRA leading to falsely high estimates of growth hormone-like activity in the serum of pregnant mice. Assuming a monovalent interaction between GH and this binding factor, the molecular weight of the latter is estimated to be 60,000 daltons. Aqueous extracts of eleven tissues, including the placenta, failed to reveal the tissue source of the GH-binding factor.