Alzheimer's disease (AD)-linked mutant amyloid precursor protein (5). A gene coding for HN was identified in a cDNA library derived from the occipital lobe of a brain from a patient with AD (5). HN is a 24-amino acid residue peptide

We have previously shown that the structural stability of humanin (HN), a neuroprotective peptide ligand, is one of the attributes to the observed activity differences between HN analogs. It has been observed that the activity increased consecutively in the S7A-HN analog, the parent HN and the S14G-HN analog, consistent with the increased stability observed in that order. In the present study, the structure and stability of another inactive analog, C8A-HN, was measured, which has been revealed to have no neuroprotective activity similar to that of the S7A-HN analog and hence may have compromised stability. While all these analogs of HN demonstrated a similar disordered secondary structure in phosphate-buffered saline at 5 ̊C, as determined by circular dichroism spectroscopy, they revealed different structures at 37 ̊C. At 37 ̊C, less active HN and inactive S7A-HN revealed a structure with a valley at ~217 nm, indicating a conversion from the disordered structure to a β-sheet. Such a conversion was largely irreversible. By contrast, C8A-HN and S14G-HN demonstrated a similar structure at 37 ̊C and at 5 ̊C and remained largely disordered. The observed small structural changes of the C8A-HN analog at 37 ̊C and its reversibility upon cooling do not support a hypothesis that the instability at 37 ̊C may have caused the reduced activity of this analog. Therefore an alternative explanation for its activity loss is required.