Conformational properties of bovine plasma albumin with a cleaved internal peptide bond.

As shown previously, proteinases frequently associated with plasma albumin samples catalyze a very limited and specific cleavage of the albumin molecule when it exists in the F conformational state near pH 3.7. The primary proteolytic product, BPA, has a molecular weight similar to or identical with that of the parent protein but yields two large fragments of molecular weight approximately 46000 and 23000 on reduction. Evidence is presented here that cleavage occurs within the disulfide loop between Cys390 and Cys434 with no detectable loss of small peptides, the amino acid composition of BPA being identical with that of the parent protein within experimental error. Cleavage exposes a new amino-terminal phenylalanine residue and may occur at the Glx392-Phe393 bond although the possibility exists that it occurs at another X-Phe bond in the unsequenced region of residues 400-402. The damaged protein has a somewhat altered secondary structure as judged from optical rotatory dispersion and circular dichroism measurements, probably an approximate 15% loss in helicity. The hydrodynamic volume is increased by approximately 20%. However, various physical studies indicate the tertiary structure to be strikingly similar to that of the native protein. Of most significance is the fact that the protein still undergoes the N-F and N-B transitions, although in both cases they occur at somewhat more moderate pH than in the parent protein. Moreover a sensitivity of the N-B transition to Ca2+ is still seen and binding behavior toward the dye 8-anilino-1-naphthalenesulfonic acid is essentially unaltered. The results are best understood in terms of the concept of a multidomain structure which has been suggested frequently for plasma ablumin. Bond cleavage damages one domain but leaves the overall structure essentially unaltered except for some weakening of the interaction between domains.