Hemopexin in the human retina: protection of the retina against heme-mediated toxicity

J Cell Physiol. 1996 Jul;168(1):71-80. doi: 10.1002/(SICI)1097-4652(199607)168:1<71::AID-JCP9>3.0.CO;2-5.


The existence of the blood-retinal barrier means that proteins that protect the retina from damage by reactive oxygen species must either be made locally or specifically transported across the barrier cells; however, such transepithelial transport does not seem to occur. Among the circulatory proteins that protect against iron-catalyzed production of free radicals are apo-transferrin, which binds ferric iron and has previously been shown to be made by cells of the neural retina (Davis and Hunt, 1993, J. Cell Physiol., 156:280-285), and the extracellular antioxidant, apo-hemopexin, which binds free heme (iron-protoporphyrin IX). Since hemorrhage and heme release can be important contributing factors in retinal disease, evidence of a hemopexin-based retinal protection system was sought. The human retina has been shown to contain apo-hemopexin which is probably synthesized locally since its mRNA can be detected in retinal tissue dissected from human donor eyes. It is likely that the retina contains a mechanism for the degradation of hemopexin-bound heme since the blood-retinal barrier also precludes the exit of heme-hemopexin from the retina. Retinal pigment epithelial cells have been found to bind and internalize heme-hemopexin in a temperature-dependent, saturable, and specific manner, analogous to the receptor-mediated endocytic system of hepatoma cells. Moreover, the binding of heme-hemopexin to the cells stimulates the expression of heme oxygenase-1, metallothionein-1, and ferritin.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Apoproteins / metabolism
  • Base Sequence
  • Cells, Cultured
  • DNA Primers / chemistry
  • Ferritins / metabolism
  • Fluorescent Antibody Technique, Indirect
  • Gene Expression
  • Heme / toxicity*
  • Heme Oxygenase (Decyclizing) / genetics
  • Hemopexin / metabolism*
  • Humans
  • Metallothionein / genetics
  • Molecular Sequence Data
  • RNA, Messenger / genetics
  • Retina / metabolism*
  • Time Factors


  • Apoproteins
  • DNA Primers
  • RNA, Messenger
  • Heme
  • Ferritins
  • Hemopexin
  • Metallothionein
  • Heme Oxygenase (Decyclizing)