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

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

Research output: Contribution to journalArticle

15 Citations (Scopus)

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.

Original languageEnglish (US)
Pages (from-to)48-54
Number of pages7
JournalExperimental Eye Research
Volume99
Issue number1
DOIs
StatePublished - Jun 2012

Fingerprint

S-Adenosylmethionine
Crystallins
Methylation
Lenses
Proteins
Cysteine
Histidine
Protein S
Alkylation
Post Translational Protein Processing
Trypsin
Digestion
Mass Spectrometry
High Pressure Liquid Chromatography

Keywords

  • Age
  • Lens protein
  • Methylation
  • Post-translational modification

ASJC Scopus subject areas

  • Ophthalmology
  • Sensory Systems
  • Cellular and Molecular Neuroscience

Cite this

Is protein methylation in the human lens a result of non-enzymatic methylation by S-adenosylmethionine? / Truscott, Roger J W; Mizdrak, Jasminka; Friedrich, Michael G.; Hooi, Michelle Y.; Lyons, Brian; Jamie, Joanne F.; Davies, Michael J.; Wilmarth, Phillip; David, Larry.

In: Experimental Eye Research, Vol. 99, No. 1, 06.2012, p. 48-54.

Research output: Contribution to journalArticle

Truscott, Roger J W ; Mizdrak, Jasminka ; Friedrich, Michael G. ; Hooi, Michelle Y. ; Lyons, Brian ; Jamie, Joanne F. ; Davies, Michael J. ; Wilmarth, Phillip ; David, Larry. / Is protein methylation in the human lens a result of non-enzymatic methylation by S-adenosylmethionine?. In: Experimental Eye Research. 2012 ; Vol. 99, No. 1. pp. 48-54.
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