in proteins that is most easily modified by oxidation. In its normal, reduced form, it has so-called non-polar or hydophobic properties. That is, it has little tendency to interact with water, a typical polar substance. The side-chain of methionine, a -CH2-S-CH3 group, is readily oxidised to the corresponding sulfoxide, -CH2-CO-CH3, which has polar properties. In that form it is very comfortable interacting with water, and less likely to interact with non-polar, "oily" environments present in a protein where it resides. The polar-vesus-nonpolar interactions are important for maintaining proper, "native" structure in proteins, so keeping methionine in its recduced, non-polar state is likely to be important for retaining the normal shape and function of the proteins in which it exists.

So, think about the normal structure of a protein as having its parts with polar amino acids on the "outside", in contact with water, whereas the parts with nonpolar amino acids down inside the middle of the structure, forming its "oily", nonpolar center.

This research involved producing transgenic drosophila (fruit flies) which had a human alpha synuclein gene added to its genome. Alpha synuclein is one of the proteins for which abnormalities in its structure or behavior (it is a major component of Lewey bodies found in PD brains) is associated with Parkinsons.
I gather that the "ectopic" (abnormal location) production of the human alpha synuclein in these insects was caused by or associated with oxidation of methionine in that or some other protein. The S-methyl cyseine compound found in garlic and cabbage apparently enhances the activity of methionine sulfoxide reductase, an enzyme that catalyzes conversion back to the "normal" reduced amino acid, presumably correcting the disrupted protein structure and preventing abnormal behavior in the flies related to the ectopically produced human alpha synuclein.

One interpretation of this result would be "Putting human alpha synuclein in fruit flies, in addition to their own alpha synuclein, gums up its nervous system in a process that involves oxidation of methionine." How this process is related to PD is not clear to me.

Robert