Spinach Protein Offers New Hope for the Blind

By:Aaron Hall

Spinach may make Popeye the Sailor Man strong, but a protein from spinach may someday strengthen the vision of people who can barely see. Researchers at Oak Ridge National Laboratory (ORNL) and the University of Southern California (USC) are investigating whether this chlorophyll-containing protein might be useful in restoring sight by replacing a key light-receiving} part of the human eye that has lost its ability to function. People who suffer from age-related macular degeneration (AMD) or retinitis pigmentosa (RP), diseases that are the leading causes of blindness worldwide, may find hope in this research.

Although the neural wiring from eye to brain is intact in patients with these diseases, their eyes lack photoreceptor activity. A team led by Elias Greenbaum at ORNL is replacing these inactive photoreceptors with a spinach protein that gives off a small electrical voltage after capturing the energy of incoming photons of light. Called Photosystem I, or PSI (pronounced PS One), the main function of this photosynthetic reaction center protein is to perform photosynthesis, using the energy of the sun to make plant tissue.

Mark Humayun of the Doheny Retina Institute at USC (www.usc.edu/hsc/doheny/) and his research team showed that if retinal tissue is stimulated electrically using pinhead-sized electrodes implanted in the eye, many patients can perceive image patterns that mimic the effects of stimulation by light. Teaming with Humayun, Greenbaum suggested the possibility of using PSI proteins to restore photoreceptor activity. ORNL experiments showed that PSI proteins can capture photon energy and generate electric voltages (about 1 V).

This project, Greenbaum stated, is based on recent original discoveries. “Using the technique of Kelvin force microscopy, we have performed the first measurements of voltages induced by photons of light from single photosynthetic reaction centers. This work was published in 2000 in an issue of the Journal of Physical Chemistry B (http://pubs.acs.org/journals/jpcbfk/). The measured photovoltage values, typically 1 V or more, are sufficiently large to trigger a neural response. We are inserting purified PSI reaction centers into retinal cells to determine whether they will restore photo-receptor function in persons who have AMD or RP. Once we demonstrate that this is possible, USC researchers will test the technique in the laboratory, and, if feasible, later in humans in clinical trials.”

In recent research, the collaborators showed that PSI reaction centers could be incorporated into a liposome, an artificial membrane made of lipids that mimics the composition of a membrane of a living cell. They also demonstrated that PSI can be functional inside a liposomethat is, it produces the experimental equivalent of a voltage when light is shined on it. A liposome probably will be used to deliver PSI to a retinal cell.

Also, in work published in Photochemistry and Photobiology in June 2001, the collaborators showed that isolated PSI reaction centers can photo-evolve hydrogen, indicating that PSI maintains its voltage-generating properties under conditions of current flow.

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