Fetal-specific forms of alpha 1-protease inhibitors in mouse plasma

Adult mouse plasma contains two distinct alpha 1-protease inhibitors (alpha 1-PIs), which we have called alpha 1-PI(E) and alpha 1-PI(T) because of their differing specificities for elastase and trypsin. Plasma levels of the two mouse inhibitors, determined either functionally or immunologically, are lower in fetal animals than in adults. Western blotting after sodium dodecyl sulfate-polyacrylamide gel electrophoresis, using specific, polyclonal antibodies raised to the inhibitors in adult plasma shows new immunoreactive species of both alpha 1-PI(T) and alpha 1-PI(E) in fetal mouse plasma with apparent molecular weights approximately 1 kDa less than the corresponding proteins in adult plasma. In both cases the fetal alpha 1-PI is replaced by the adult protein between 2 and 6 days after birth. After chemical deglycosylation, the molecular weight difference between the adult and fetal forms of alpha 1-PI(E) and alpha 1-PI(T) is retained, suggesting that the adult and fetal mouse alpha 1-PIs have different amino acid sequences. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the protease inhibitors, cleaved specifically at the protease binding site, indicated that the difference in sequence between the adult and fetal proteins resided in the 50-60 kDa N-terminal fragment.