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α₁-Microglobulin is a 26-kd protein, widespread in plasma and tissues and well-conserved among vertebrates. α₁-Microglobulin belongs to the lipocalins, a protein superfamily with highly conserved 3-dimensional structures, forming an internal ligand binding pocket. The protein, isolated from urine, has a heterogeneous yellow-brown chromophore bound covalently to amino acid side groups around the entrance of the lipocalin pocket. α₁-Microglobulin is found in blood both in free form and complex-bound to immunoglobulin A (IgA) via a half-cystine residue at position 34. It is shown here that an α₁-microglobulin species, which we name t–α₁-microglobulin (t = truncated), with a free Cys34 thiol group, lacking its C-terminal tetrapeptide, LIPR, and with a more polar environment around the entrance of the lipocalin pocket, is released from IgA–α₁-microglobulin as well as from free α₁-microglobulin when …