THE FATTY-ACID ANALOG 11-(DANSYLAMINO)UNDECANOIC ACID IS A FLUORESCENT-PROBE FOR THE BILIRUBIN-BINDING SITES OF ALBUMIN AND NOT FOR THE HIGH-AFFINITY FATTY ACID-BINDING SITES

1. The fluorescent fatty acid probe 11-(dansylamino)undecanoic acid (DAUDA) binds with high affinity to bovine and human serum albumin (BSA and HSA) at three sites. 2. The K(d) of the primary binding site could not be determined; however, the two secondary sites appeared to be equivalent, with an apparent K(d) of 8 x 10-7 M for both BSA and HSA. 3. The spectral characteristics of DAUDA when bound to the primary site of the two albumins were different, with HSA producing a greater fluorescence enhancement and emission maximum at a shorter wavelength (480 nm) than for BSA (495 nm). 4. Displacement studies indicated that the DAUDA-binding sites were not equivalent to the primary long-chain fatty acid-binding sites on albumin, but corresponded to the bilirubin sites. Fatty acyl-CoAs also bind to the bilirubin sites, as do medium-chain fatty acids. 5. The solubility, stability and spectral properties of DAUDA make it an excellent probe for investigating the bilirubin-binding sites of albumin, particularly HSA.