Aggrelyte‐2 promotes protein solubility and decreases lens stiffness through lysine acetylation and disulfide reduction: Implications for treating presbyopia

Aging proteins in the lens become increasingly aggregated and insoluble, contributing to presbyopia. In this study, we investigated the ability of aggrelyte‐2 (N,S‐diacetyl‐L‐cysteine methyl ester) to reverse the water insolubility of aged human lens proteins and to decrease stiffness in cultured human and mouse lenses. Water‐insoluble proteins (WI) of aged human lenses (65–75 years) were incubated with aggrelyte‐2 (500 μM) for 24 or 48 h. A control compound that lacked the S‐acetyl group (aggrelyte‐2C) was also tested. We observed 19%–30% solubility of WI upon treatment with aggrelyte‐2. Aggrelyte‐2C also increased protein solubility, but its effect was approximately 1.4‐fold lower than that of aggrelyte‐2. The protein thiol contents were 1.9‐ to 4.9‐fold higher in the aggrelyte‐2‐ and aggrelyte‐2C‐treated samples than in the untreated samples. The LC–MS/MS results showed N ( ε )‐acetyllysine (AcK) levels of 1.5 to 2.1 nmol/mg protein and 0.6 to 0.9 nmol/mg protein in the aggrelyte‐2‐ and aggrelyte‐2C‐treated samples. Mouse (C57BL/6J) lenses (incubated for 24 h) and human lenses (incubated for 72 h) with 1.0 mM aggrelyte‐2 showed significant decreases in stiffness with simultaneous increases in soluble proteins (human lenses) and protein‐AcK levels, and such changes were not observed in aggrelyte‐2C‐treated lenses. Mass spectrometry of the solubilized protein revealed AcK in all crystallins, but more was observed in α‐crystallins. These results suggest that aggrelyte‐2 increases protein solubility and decreases lens stiffness through acetylation and disulfide reduction. Aggrelyte‐2 might be useful in treating presbyopia in humans.