Interaction of bilirubin with reconstituted collagen fibrils

Mapping the ligand-binding sites and disease-associated mutations on the most abundant protein in the human, type I collagen

Type I collagen is the most abundant protein in humans, and it helps to maintain the integrity of many tissues via its interactions with cell surfaces, other extracellular matrix molecules, and growth and differentiation factors. Nearly 50 molecules have been found to interact with type I collagen, and for about half of them, binding sites on this collagen have been elucidated. In addition, over 300 mutations in type I collagen associated with human connective tissue disorders have been described. However, the spatial relationships between the known ligand-binding sites and mutation positions have not been examined. To this end, here we have created a map of type I collagen that includes all of its ligand-binding sites and mutations. The map reveals the existence of several hot spots for ligand interactions on type I collagen and that most of the binding sites locate to its C-terminal half. Moreover, on the collagen fibril some potentially relevant relationships between binding sites were observed including the following: fibronectin- and certain integrin-binding regions are near neighbors, which may mechanistically relate to fibronectin-dependent cell-collagen attachment; proteoglycan binding may potentially impact upon collagen fibrillogenesis, cell-collagen attachment, and collagen glycation seen in diabetes and aging; and mutations associated with osteogenesis imperfecta and other disorders show apparently nonrandom distribution patterns within both the monomer and fibril, implying that mutation positions correlate with disease phenotype. These and other observations presented here may provide novel insights into evaluating type I collagen functions and the relationships between its binding partners and mutations.

The structural characterization and bilirubin-binding properties of albumin Herborn, a [Lys240-->Glu] albumin mutant

We report the molecular defect of albumin Herborn, a new genetic variant of human serum albumin which has been found in Germany. Isoelectric focusing analysis of CNBr fragments from the purified variant allowed us to localize the mutation in fragment CNBr 3 (residues 124-298). This fragment was isolated on a preparative scale and subjected to tryptic and V8 protease digestion. Sequence determination of the abnormal tryptic and V8 peptides revealed that the variant arises from the substitution Lys240-->Glu. The -2 charge change of albumin Herborn, which is probably due to a A-->G transition in the first position of the corresponding codon in the structural gene, has no significant effect on its electrophoretic mobility under non-denaturating conditions. Therefore we have assumed that residue 240, which has been implicated in the bilirubin primary binding site (Jacobsen, C. (1978) Biochem. J. 171, 453-459), is buried. The binding of bilirubin and biliverdin by albumin Herborn was quantified using the fluorescence quenching method. The apparent equilibrium association constants (Ka +/- SD) and the number of high-affinity binding sites (n) of the defatted variant for bilirubin and biliverdin were Ka = 1.03 +/- 0.18 x 10(8) M-1, n = 1.07; and Ka = 7.48 +/- 1.10 x 10(6) M-1, n = 1.01, respectively. The Ka values are about 93.3% and 99.1% of the values found for the normal protein under the same conditions. These results strongly suggest that Lys240 of human serum albumin is not the basic residue involved in ion pairing with one of the carboxylate groups of bilirubin at its high-affinity site.

Bilirubin encephalopathy: changing concepts

This overview begins with a discussion of clinical approaches to prevent brain damage in jaundiced newborns. Results of longitudinal follow-up studies are given. The main part deals with pathogenesis, whereby bilirubin albumin binding, competition by drugs, bilirubin distribution and equilibration in the body, the increased sensitivity of the infantile nervous system, the mechanism of bilirubin entry into nerve cells and the resulting intracellular damage are emphasized.