Lens transglutaminase and cataract formation

Area of Science:

• Ophthalmology
• Biochemistry
• Enzymology

Background:

• Human cataractous protein exhibits elevated levels of gamma-glutamyl-epsilon-lysine isopeptides.
• These isopeptides are indicative of enzymatic crosslinking activity within the lens.
• The presence of transglutaminase (TGase) and endogenous protein substrates in the lens supports this hypothesis.

Purpose of the Study:

• To investigate the role of transglutaminase (TGase) in the formation of gamma-glutamyl-epsilon-lysine isopeptides in human cataract.
• To characterize the properties and localization of TGase activity within the mammalian lens.
• To explore the potential involvement of TGase-mediated protein crosslinking in cataractogenesis.

Main Methods:

• Analysis of protein polymers from human cataract for gamma-glutamyl-epsilon-lysine isopeptide content.
• Biochemical characterization of lens transglutaminase (TGase), including calcium (Ca2+) dependence.
• Incubation of rabbit lens cortex homogenate with radioactive ([14C]putrescine) or fluorescent (dansylcadaverine) amine substrates in the presence of Ca2+.

Main Results:

• Significant amounts of gamma-glutamyl-epsilon-lysine isopeptides were detected in human cataract protein.
• Lens transglutaminase (TGase) activity was confirmed to be Ca2+ dependent and present in the lens cortex.
• Selective incorporation of amine substrates into heavier subunits (Mr ~26,000 and 30,000) of beta-crystallins was observed, indicating TGase targets these proteins.

Conclusions:

• Transglutaminase (TGase) activity, leading to gamma-glutamyl-epsilon-lysine isopeptide formation, is implicated in human cataract.
• The enzyme's activity is calcium-dependent and localized primarily in the lens cortex.
• TGase-mediated crosslinking of beta-crystallin subunits may contribute to the structural changes observed in cataract.