| Abstract: | Twelve fragments of bovine serum albumin, isolated following limited tryptic or peptic hydrolysis, have been studied to define secondary structure and locate ligand-binding sites. Based on circular dichroism, the conformational pattern of albumin (68% alpha helix and 18% beta structure) is substantially retained by individual fragments, indicating that secondary configuration is locally determined and is not destroyed during the cleavage process nor during fragment purification. The strong bilirubin-binding site of bovine serum albumin is present in 3 of the 12 fragments. Residues 186-238 are common to the three fragments and absent from those fragments which do not bind bilirubin; consequently the strong bilirubin-binding site is suggested to involve this region. By similar reasoning, the presence of palmitate-binding sites in some fragments and not in others indicates that the three strongest sites for the binding of palmitate are located in the carboxyl-terminal two-thirds of the molecule. The first site (KA approximately 2 X 10(7) M-1) is suggested as residues 377-503; the second site (KA approximately 8 X 10(6) M-1), residues 239-306; the third site (KA approximately 2 X 10(6) M-1), residues 307-377. Bromocresol Green, a reagent used in the assay of ablumin, was bound by fragments rougly in proportion to their size but showed particular affinity for the region of the strong bilirubin-binding site. The fluorescent probe, 8-anilino-1-naphthalensulfonate, was in general bound by large fragments, supporting the concept that this ligand is held principally in clefts between domains of the macromolecule. |
