LOCATION OF A NOVEL TYPE OF INTERPOLYPEPTIDE CHAIN LINKAGE IN THE HUMAN PROTEIN HC-IGA COMPLEX (HC-IGA) AND IDENTIFICATION OF A HETEROGENEOUS CHROMOPHORE ASSOCIATED WITH THE COMPLEX

Protein HC (human complex-forming glycoprotein, heterogeneous in charg e) is a member of the lipocalin superfamily of hydrophobic ligand-bind ing proteins. The quantitatively dominating blood plasma form of prote in HC is a protein HC-IgA complex (HC-IgA), which is the fourth most a bundant immunoglobulin species in plasma. A photodiode array detection system on-line with a high performance liquid chromatograph has allow ed the identification of low amounts of a heterogeneous fluorescent ch romophore covalently bound to HC-IgA, and displaying significant absor ption in the visible region in resemblance to the free protein HC chro mophore. Several structurally related chromophore-containing linked pe ptides, carrying 80% of the light absorption at 330 nm of HC-IgA, were isolated from a pepsin-produced protein HC-alpha1-nonapeptide. Sequen ce analysis of these linked peptides demonstrated that the bond betwee n protein HC and IgA represents a novel type of reduction-resistant li nkage between polypeptide chains and involves the cysteine residue 34 of protein HC and the penultimate cysteine residue of the carboxyl-ter minal part of one of the IgA heavy chains, as well as the heterogeneou s fluorescent chromophore. The light absorption and fluorescent spectr a of the chromophore-linked peptides were similar to those of native f ree protein HC.