Evaluation of a serum protein in pregnant rats

The increase of a serum protein in the circulation of rats during pregnancy has been described. The protein is different from the human pregnancy zone protein previously described. This new serum protein factor is present in normal rat serum and accounts for approximately 1% of the total serum protein. The protein factor disappears from the serum of animals bearing Morris hepatomas 7777 and 7800. The amount of factor diminishes as the tumors increase in size. The levels of this protein in the serum increased several fold during pregnancy with highest levels found shortly before parturition. Levels remained elevated for at least 12 days post partum. No corresponding increase in the levels of this serum factor in the fetal circulation was observed. Increased levels in the maternal circulation were due to an increased synthesis of this serum protein during pregnancy. A corresponding band was observed on polyacrylamide gels for normal human female and pregnant human sera or plasma. However, there was no increase during pregnancy as judged by polyacrylamide gel electrophoresis. Quantitative determination of the protein in human sera was not possible since the band corresponding to the serum protein in rats did not cross react with a rabbit anti-rat serum factor.