Multiple forms of rat-serum alpha 1-protease inhibitor. Involvement of sialic acid in the multiplicity of three original forms.

alpha 1-Protease inhibitor was purified from rat serum by two different methods, of which an immunoaffinity method should be preferentially used to obtain all of the multiple forms. The protein thus obtained showed a single protein band in dodecylsulphate/polyacrylamide gel electrophoresis corresponding to a molecular weight of 54000, and contained 13.2% carbohydrate by weight. By column isoelectric focusing in a pH 3.5-5.0 gradient the purified alpha 1-protease inhibitor was separated into five forms with pI values from 4.3-4.7. The amino acid composition of each form was identical, while sialic acid content was significantly different from each other. The most acidic form contained 6.7 residues/molecule, the most basic form, 5.1 residues/molecule, and three forms between them showed proportionally intermediate values between the two. When alpha 1-protease inhibitor was treated with neuraminidase, the five forms were converted finally into three major forms with pI values of 5.3-5.7. In addition, the major form (band 3) of the inhibitor was also converted into three forms after complete removal of sialic acid. These results suggest that alpha 1-protease inhibitor originally exists as three forms with different charges, possibly due to modification of amino acids which might not be detectable by the amino acid composition analysis in the present study. A possible explanation was presented for involvement of sialic acid in appearance of multiple forms originating from three parental forms.