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Thiol-dependent antioxidant activity of interphotoreceptor retinoid-binding protein

Federico Gonzalez-Fernandez1, Dongjin Sung2, Karen M Haswell3

  • 1Medical Research Service, Veterans Affairs Medical Center, Buffalo, NY, USA; Departments of Ophthalmology (Ross Eye Institute) and Pathology & Anatomic Sciences, the State University of New York, Buffalo, NY, USA; SUNY Eye Institute, State University of New York, NY, USA.

Experimental Eye Research
|January 16, 2014
PubMed

Insights

Interphotoreceptor retinoid-binding protein (IRBP) demonstrates significant antioxidant activity, acting as a free radical scavenger. This newly identified function of IRBP is crucial for maintaining the redox balance within the subretinal space.

Area of Science:

  • Biochemistry
  • Ophthalmology
  • Molecular Biology

Background:

  • Interphotoreceptor retinoid-binding protein (IRBP) is essential for photoreceptor survival and function.
  • IRBP contains multiple cysteine residues, with some potentially existing as free thiols.
  • Historically, IRBP purification has been challenging due to protein instability.

Purpose of the Study:

  • To investigate the potential functions of free thiols in IRBP.
  • To explore the antioxidant properties of purified bovine IRBP (bIRBP).
  • To elucidate the role of IRBP in the subretinal space redox environment.

Main Methods:

  • Purification of bIRBP using concanavalin A, ion exchange, and size exclusion chromatography.
  • Assay of antioxidant activity by measuring inhibition of 2,2'-azinobis [3-ethylbenzothiazoline-6-sulfonate] oxidation.
  • Homology modeling to predict the structural relationship between cysteine residues and retinoid binding sites.
  • Chemical modification of free cysteines using N-ethylmaleimide.

Main Results:

  • Purified bIRBP exhibited potent free radical scavenging activity, exceeding that of ovalbumin, thioredoxin, and Trolox.
  • Alkylation of free cysteines abolished antioxidant activity but did not affect all-trans retinol binding.
  • Structural modeling suggested Cys 1051 is positioned near a ligand-binding site.

Conclusions:

  • IRBP possesses significant antioxidant capabilities, functioning as a free radical scavenger.
  • The free thiol groups of IRBP are critical for its antioxidant function.
  • IRBP plays a novel role in regulating the redox state of the subretinal space.

Keywords:
X-ray structurecrystallographyinterphotoreceptor matrixinterphotoreceptor-retinoid binding proteinphotoreceptorsretinaretinoid binding proteinvisual cycle

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