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

Research output: Contribution to journalJournal articleResearchpeer-review

Roger J W Truscott, Jasminka Mizdrak, Michael G Friedrich, Michelle Y Hooi, Brian Lyons, Joanne F Jamie, Michael Jonathan Davies, Phillip A Wilmarth, Larry L David

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.

Original languageEnglish
JournalExperimental Eye Research
Volume99
Pages (from-to)48-54
Number of pages7
ISSN0014-4835
DOIs
Publication statusPublished - Jun 2012
Externally publishedYes

    Research areas

  • Adolescent, Adult, Aged, Aging, Chromatography, High Pressure Liquid, Cysteine, Histidine, Humans, Lens, Crystalline, Mass Spectrometry, Methylation, Middle Aged, Protein Methyltransferases, Protein Processing, Post-Translational, Proteomics, S-Adenosylmethionine, Tissue Donors, Young Adult, beta-Crystallin A Chain

ID: 128974982