Is protein methylation in the human lens a result of non-enzymatic methylation by S-adenosylmethionine?

Abstract

Since crystallins in the human lens do not turnover, they are susceptible to modification by reactive molecules over time. Methylation is a major post-translational lens modification, however the source of the methyl group is not known and the extent of modification across all crystallins has yet to be determined. Sites of methylation in human lens proteins were determined using HPLC/mass spectrometry following digestion with trypsin. The overall extent of protein methylation increased with age, and there was little difference in the extent of modification between soluble and insoluble crystallins. Several different cysteine and histidine residues in crystallins from adult lenses were found to be methylated with one cysteine (Cys 110 in γD crystallin) at a level approaching 70%, however, methylation of crystallins was not detected in fetal or newborn lenses. S-adenosylmethionine (SAM) was quantified at significant (10–50 μM) levels in lenses, and in model experiments SAM reacted readily with N-α-tBoc-cysteine and N-α-tBoc-histidine, as well as βA3-crystallin. The pattern of lens protein methylation seen in the human lens was consistent with non-enzymatic alkylation. The in vitro data shows that SAM can act directly to methylate lens proteins and SAM was present in significant concentrations in human lens. Thus, non-enzymatic methylation of crystallins by SAM offers a possible explanation for this major human lens modification.

Oxidative photodegradation of ocular tissues: Beneficial effects of filtering and exogenous antioxidants

“…As such, any damage to fiber cells within the lens simply accumulates with age, causing the lens to slowly opacify. Transamidation, methylation, and oxidation account for the major post-translational modifications of lens proteins (Andley et al, 2011;Beebe et al, 2011;Michael and Bron, 2011;Sweeney and Truscott, 1998;Truscott et al, 2012;Lampi et al, 2014). This opacification is both distinctive and relatively stable across the lifespan.…”

“…Although predominately visible light reaches the retina, a small (~1%) amount of UV radiation does actually penetrate to the adult retina (Weale, 1988; van de Kraats and van Norren, 2007a). 2 Werner and Spillmann (1989) described the very negative effects of UV radiation on the retina.…”

“…Fig. 15, compiled via a meta-analysis (Glickman, 2011), illustrates the action spectrum for retinal damage via exposure to electromagnetic radiation in the UV/blue light region (see also van Norren and Gorgels, 2011). The chromophore is the bleach product of rhodopsin.…”

The etiology of human age-related cataract. Proteins don't last forever

“…In the human lens the major PTM due to reaction with biochemicals appears to be methylation [9, 10] and the source is S-adenosyl methionine [10]. Given the sustained high levels of glucose in the lens, the levels of AGEs, such as carboxymethyl or carboxyethyl Lys and Arg, are much less e.g.…”

Advances in purification and separation of posttranslationally modified proteins

“…Methylation is often catalyzed by methyltransferase enzymes that utilize S-adenosylmethionine as a substrate, but S-adenosylmethionine can also readily methylate cysteine and histidine residues non-enzymatically in vitro, and a similar non-enzymatic mechanism has been proposed for the methylation of some proteins in vivo, e.g. human lens crystallins [231]. It was long believed that N-methylation is irreversible and that reversibility of O-methylation is due to spontaneous non-enzymatic decomposition of the ester bond.…”