Intrinsic fluorescence study of the interaction of human apolipoprotein H with phospholipid vesicles

Apolipoprotein H (ApoH) is a plasma glycoprotein with its in vivo physiolog ical and pathogenic roles being closely related to its interaction with neg atively charged membranes. In this paper, the interaction of ApoH with phos pholipid vesicles was characterized by (i) detecting the wavelength shift o f the fluorescence spectrum of ApoH and (ii) measuring the fluorescence que nching extent of ApoH by the membrane resident quencher 1-palmitoyl-2-stear oyl-(5-doxyl)-sn-glycero-3-phosphocholine (DPC). The observed blue shift up on addition of DMPG vesicles indicated that the tryptophan residues of ApoH moved from a polar to a nonpolar environment. The insertion ability of Apo H into PG-containing vesicles did not depend on the PG content in a stoichi ometric way as did the blue shift, indicating that the negatively charged D MPG does not serve as a specific binding site but rather provides a suitabl e microenvironment for ApoH interaction, The finding that the detachment ef fect of cations on the blue shift is remarkably different from that on the quenching extent suggests that ApoH is capable of existing in two different conformations when membrane-bound.