Sequence comparison of human plasma alpha1-acid glycoprotein and the immunoglobulins

The amino acid sequence of human plasma alpha1-acid glycoprotein, upon comparison with the sequences of other blood proteins, was shown to possess significant similarity with the immunoglobulins. Employing direct and corrected sequence identity, the average mutation value and two different computer comparisons for the evaluation of sequence similarity, the following two regions of this alpha-globulin, which account for approximately half of the total amino acid sequence of the protein, were found to possess sequence similarity with the immunoglobulins. a) The region from residues 77 through 125 proved to be related to the variable region of several human H and L chains, and b) the region from residues 136 through 166 was found to be related not only to the constant region of a human and a mouse L chain but also to the third and fourth constant region of a rabbit and a human H chain, respectively. These results suggest that alpha1-acid glycoprotein is probably related to the immunoglobulins and further suggest that it possibly diverged from the immunoglobulin evolutionary tree prior to the formation of the primitive L chain.