Human plasma low density lipoprotein: a fluorescence study.

Hydrophobic nature of human plasma low density lipoprotein surface has been studied by fluorescence spectroscopic method. Enhancement in 8-anilino-1-naphthalene sulphonate (ANS) fluorescence quantum yield from 0.004 to 0.114 at 470 nm has suggested that the ANS binding sites are fairly low in polarity. LDL has been found to have 77 homogeneous binding sites for ANS (Ka = 2.5 x 10(5) M-1). The binding of an ANS molecule does not affect the successive binding sites. Variation in temperature from 15 degrees to 45 degrees C did neither alter the number of binding sites nor association constant. Quenching of protein fluorescence (lambda exc 286 nm, lambda ems 336 nm) indicated the occurrence of energy transfer in LDL-ANS complex arising from conformational changes capable of bringing charge acceptor segments near to other ANS site. About 30-fold increase in ANS quantum yield and large shift in the emission maximum are characteristic features of large hydrophobic environment on the surface of LDL particle.