Conversion of human alpha-lactalbumin to an apo-like state in the complexes with basic poly-amino acids: toward understanding of the molecular mechanism of antitumor action of HAMLET

It was recently shown that alpha-lactalbumin associated with oleic acid (HAMLET) interacts with core histones thereby triggering apoptosis of tumor cells (J. Biol. Chem. 2003, 278, 42131). In previous work, we revealed that monomeric alpha-lactalbumin in the absence of fatty acids can also interact with histones and, moreover, with basic poly-amino acids (poly-Lys and poly-Arg) that represent simple models of histone proteins (Biochemistry 2004, 43, 5575). Association of alpha-lactalbumin with histone or poly-Lys(Arg) essentially changes its properties. In the present work, the character of the changes in structural properties and conformational stability of alpha-lactalbumin in the complex with poly-Lys(Arg) has been studied in detail by steady-state fluorescence, circular dichroism, and differential scanning calorimetry. Complex formation strongly depends on ionic strength, confirming its electrostatic nature. Experiments with the poly-amino acids of various molecular masses demonstrated a direct proportionality between the number of alpha-lactalbumin molecules bound per poly-Lys(Arg) and the surface area of the poly-amino acid random coil. The binding of the poly-amino acids to Ca2+-saturated human alpha-lactalbumin decreases its thermal stability down to the level of its free apo-form and decreases Ca2+-affinity by 4 orders of magnitude. The conformational state of alpha-lactalbumin in a complex with poly-Lys(Arg), named alpha-LActalbumin Modified by Poly-Amino acid (LAMPA), differs from all other alpha-lactalbumin states characterized to date, representing an apo-like (molten globule-like) state with substantially decreased affinity for calcium ion. The requirement for efficient conversion of alpha-lactalbumin to the LAMPA state is a poly-Lys(Arg) chain consisting of several tens of amino acid residues.