Preparation and Partial Structural Characterization of α1T-Glycoprotein from Normal Human Plasma

α1T-glycoprotein (α1T) was isolated from normal human plasma in the immunochemically homogeneous state. The partial amino acid sequence and carbohydrate chains of this glycoprotein were determined. To achieve this, the carboxymethylated α1T was analyzed by sequencing some of the lysylendoprotease, V8 protease, tryptic, and cyanogen bromide peptides as well as the N-terminal sequence of the protein. A large number of amino acid residues (460 amino acids) was determined by chemical procedure. The peptide sequences were compared with that of other proteins. A high degree of homology was found for proteins of the albumin family. Further, human α-albumin, a new member of this protein family, showed an amino acid sequence identical to that of α1T indicating that these two proteins are very similar in amino acid sequence and composition. These proteins are closely related to α-fetoprotein; however, five carbohydrate chains were found on α1T at Asn12, Asn88, Asn362, Asn381, and Asn467 as biantennary complex type chains and the chain on Asn362 possessed a rare consensus sequence of Asn-X-Cys. Thus, α1T distinguishes itself by possessing five N-glycans, a finding reported here for the first time for the ALB family.