We can see this simply by looking at human blood, where over 90 percent of the free cysteine exists as its oxidized disulfide form (9)

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Reactive oxygen species in inflammation and tissue injury
Also, some research suggests that it crosses the blood-brain barrier more easily
doi: 10.1111/pcmr.12941 16 ZhaoYZhangCFRossiterHEckhartLKonigUKarnerSet al