Interactions of alpha-lactalbumins with lipid vesicles studied by tryptophan fluorescence

Complexes of alpha-lactalbumin (alpha-LA)1 with dimyristoylphosphatidylcholine (DMPC) or dipalmitoylphosphatidylcholine (DPPC) liposomes at pH 8 and at pH 2 have been obtained by means of gel filtration. Thermal denaturation of alpha-LA complexes of DMPC or DPPC at pH 8 was found to depend on the saturation of protein by metal cations. The intrinsic fluorescence of DMPC-alpha-LA and DPPC-alpha-LA was sensitive to two thermal transitions. The first transition corresponded to the Tc of the lipid vesicles, while the second transition arose from the denaturation of the protein. Fluorescence spectrum position suggested that at low temperature tryptophan accessibility increases upon protein-DMPC or protein-DPPC association. At temperatures above the protein transition (70 degrees C) tryptophan appears to interact significantly with the apolar phase of DMPC and DPPC, evidenced by spectral blue shifts. Whereas the free protein at pH 2 adopts the molten globule (MG) state and is characterized by the absence of a thermal transition, the rapidly-isolated DMPC-alpha-LA complex was characterized by the appearance of a distinct fluorescence thermal transition between 50 and 60 degrees C. This result is consistent with a model of a partially-inserted form of alpha-LA which may possess some degree of tertiary structure and therefore unfolds cooperatively.