In this study ␣-lactalbumin was converted from the regular, native state to a folding variant with altered biological function. The folding variant was shown to induce apoptosis in tumor cells and immature cells, but healthy cells were resistant to this effect. Conversion to HAMLET (human ␣-lactalbumin made lethal to tumor cells) required partial unfolding of the protein and a specific fatty acid, C18:1, as a necessary cofactor. Conversion was achieved with ␣-lactalbumin derived from human milk whey and with recombinant protein expressed in Escherichia coli. We thus have identified the folding change and the fatty acid as two key elements that define HAMLET, the apoptosis-inducing functional state of ␣-lactalbumin. Although the environment in the mammary gland favors the native conformation of ␣-lactalbumin that serves as a specifier in the lactose synthase complex, the conditions under which HAMLET was formed resemble those in the stomach of the nursing child. Low pH is known to release Ca 2؉ from the highaffinity Ca 2؉ -binding site and to activate lipases that hydrolyze free fatty acids from milk triglycerides. We propose that this single amino acid polypeptide chain may perform vastly different biological functions depending on its folding state and the in vivo environment. It may be speculated that molecules like HAMLET can aid in lowering the incidence of cancer in breast-fed children by purging of tumor cells from the gut of the neonate.

This study characterized a protein complex in human milk that induces apoptosis in tumor cells but spares healthy cells. The active fraction was purified from casein by anion exchange chromatography. Unlike other casein components the active fraction was retained by the ion exchanger and eluted after a high salt gradient. The active fraction showed N-terminal amino acid sequence identity with human milk ␣-lactalbumin and mass spectrometry ruled out post-translational modifications. Size exclusion chromatography resolved monomers and oligomers of ␣-lactalbumin that were characterized using UV absorbance, fluorescence, and circular dichroism spectroscopy. The high molecular weight oligomers were kinetically stable against dissociation into monomers and were found to have an essentially retained secondary structure but a less well organized tertiary structure. Comparison with native monomeric and molten globule ␣-lactalbumin showed that the active fraction contains oligomers of ␣-lactalbumin that have undergone a conformational switch toward a molten globule-like state. Oligomerization appears to conserve ␣-lactalbumin in a state with molten globule-like properties at physiological conditions. The results suggest differences in biological properties between folding variants of ␣-lactalbumin.

Treatment with topical alpha-lactalbumin-oleic acid has a beneficial and lasting effect on skin papillomas.

Proteins can adjust their structure and function in response to shifting environments. Functional diversity is created not only by the sequence but by changes in tertiary structure. Here we present evidence that lipid cofactors may enable otherwise unstable protein folding variants to maintain their conformation and to form novel, biologically active complexes. We have identified unsaturated C18 fatty acids in the cis conformation as the cofactors that bind apo ␣-lactalbumin and form HAMLET (human ␣-lactalbumin made lethal to tumor cells). The complexes were formed on an ion exchange column, were stable in a molten globule-like conformation, and had attained the novel biological activity. The protein-fatty acid interaction was specific, as saturated C18 fatty acids, or unsaturated C18:1trans conformers were unable to form complexes with apo ␣-lactalbumin, as were fatty acids with shorter or longer carbon chains. Unsaturated cis fatty acids other than C18:1:9cis were able to form stable complexes, but these were not active in the apoptosis assay. The results demonstrate that stereo-specific lipid-protein interactions can stabilize partially unfolded conformations and form molecular complexes with novel biological activity. The results offer a new mechanism for the functional diversity of proteins, by exploiting lipids as essential, tissue-specific cofactors in this process.

HAMLET (human ␣-lactalbumin made lethal to tumor cells) is a complex of human ␣-lactalbumin and oleic acid (C18:1:9 cis) that kills tumor cells by an apoptosis-like mechanism. Previous studies have shown that a conformational change is required to form HAMLET from ␣-lactalbumin, and that a partially unfolded conformation is maintained in the HAMLET complex. This study examined if unfolding of ␣-lactalbumin is sufficient to induce cell death. We used the bovine ␣-lactalbumin Ca 2+ site mutant D87A, which is unable to bind Ca 2+ , and thus remains partially unfolded regardless of solvent conditions. The D87A mutant protein was found to be inactive in the apoptosis assay, but could readily be converted to a HAMLET-like complex in the presence of oleic acid. BAMLET (bovine ␣-lactalbumin made lethal to tumor cells) and D87A-BAMLET complexes were both able to kill tumor cells. This activity was independent of the Ca 2+ site, as HAMLET maintained a high affinity for Ca 2+ but D87A-BAMLET was active with no Ca 2+ bound. We conclude that partial unfolding of ␣-lactalbumin is necessary but not sufficient to trigger cell death, and that the activity of HAMLET is defined both by the protein and the lipid cofactor. Furthermore, a functional Ca 2+ -binding site is not required for conversion of ␣-lactalbumin to the active complex or to cause cell death. This suggests that the lipid cofactor stabilizes the altered fold without interfering with the Ca 2+ site.