Occurrence and Molecular Weight of Rabbit Uterine "Blastokinin"

The presence of a specificprotein occurring in uterine flushings between Days 3 and 8 of pregnancy in the rabbit has been confirmed by disc gel electrophoresis and gel filtration over Sephadex. The identity of this protein with “blastokinin” was established by similarity of electrophoretic mobility, elution profile on gel filtration, amino acid composition, and time of occurrence. The molecular weight of “blastokinin” was estimated as 14,525 by gel filtration and 10,045 by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, confirming recent reports that the original value of 27,000 was overestimated. The difference between the two methods of determination suggests that “blastokinin” may exist as a dimer. Guinea pig anti-”blastokinin” antiserum after absorption with rabbit serum albumin was shown to be specific for “blastokinin” by immunoelectrophoresis. The absorbed antiserum showed the presence of a protein similar to “blastokinin” in uterine fiushings from nonpregnant rabbits. It appears that “blastokinin” is present in minute quantities in nonpregnant rabbits and that its secretion is greatly accelerated during early pregnancy.