N-Homocysteinyl-Proteins - Homocysteine in Protein Structure/Function and Human Disease - page 88

5.4.5.3 N -Hcy-Prion

-helical prion protein (PrP C ) into a Misfolding of the natively -sheet-rich isoform

(PrP Sc ) and the formation of amyloid aggregates are believed to be the causes of
neurodegenerative diseases such as scrapie in sheep, bovine spongiform encepha-
lopathy, and hereditary Creutzfeldt-Jakob disease and Gerstmann-Straeus-
sler-Scheinker-like syndrome in humans. The form of disease is determined by
two prion types, which differ in their stabilities against denaturing agents, have
different proteinase K cleavage sites, and form different prion aggregate deposits
[345]. Multiple factors, including genetic variation, physicochemical environment,
and posttranslational modifications (such as glycation) [346] determine the propen-
sity of PrP C to form aggregates.
A recent study suggests that N-homocysteinylation contributes to the conversion
of PrP C to PrP Sc [347]. Incubation of ovine PrP C with Hcy-thiolactone leads to N-
homocysteinylation of the residues Lys197 and Lys207 and the formation of prion
multimers, detected on nonreducing SDS-PAGE gels. The DLS measurements
reveal large N-Hcy-PrP Sc aggregates that enhance thioflavin T fluorescence and
are resistant to proteinase K digestion. Epifluorescence microscopy in the presence
of thioflavin T shows that these aggregates have cluster-like structure. Infrared
spectroscopy reveals increased content of

α β

-sheet structures in N-Hcy-PrP Sc relative
to unmodified PrP C . Taken together, these findings show that N-homocysteinylation
accelerates amyloid-like transformation of PrP C [347].

β

5.4.5.4 N -Hcy-Amyloid -Peptide

β

Genetic hyperhomocysteinemia causes neurological abnormalities in humans,
manifested by seizures and mental retardation [20]. In a general population, ele-
vated plasma Hcy level is associated with neurodegenerative diseases such as
dementia and Alzheimer's disease [56].
Amyloid β -peptide (A β ) is a normal product of cellular metabolism, and

aggregation of A β plays central role in mediating neurotoxicity in Alzheimer's

disease. A β is natively unfolded in the monomeric state. In vitro modification of

A β 1-42 with Hcy-thiolactone decreases its propensity to form amyloid fibrils

[348]. CD spectroscopy reveals attenuated changes in the signal at 218 nm
obtained with N-Hcy-A β 1-42 , suggesting that N-homocysteinylation inhibits

β β 1-42 becomes more toxic and induces cell death in

PC12 cells to a greater extent than unmodified A

-sheet formation. N-Hcy-A

β 1-42 does. These data suggest

that -sheet structures

decreases oligomer elongation and fibril formation and leads to new protofibrils
with low self-assembly and high toxicity [348].

the diminished propensity of N-Hcy-A β 1-42 to form β