Lens α-Crystallin and Hypericin: A Photophysical Mechanism Explains Observed Lens Damage†¶

Hypericin-mediated photooxidative damage of α-crystallin in human lens epithelial cells

St. John's wort (Hypericum perforatum), a perennial herb native to Europe, is widely used for and seems to be effective in treatment of mild to moderate depression. Hypericin, a singlet oxygen-generating photosensitizer that absorbs in both the visible and the UVA range, is considered to be one of the bioactive ingredients of St. John's wort, and commercial preparations are frequently calibrated to contain a standard concentration. Hypericin can accumulate in ocular tissues, including lenses, and can bind in vitro to α-crystallin, a major lens protein. α-crystallin is required for lens transparency and also acts as a chaperone to ensure its own integrity and the integrity of all lens proteins. Because there is no crystallin turnover, damage to α-crystallin is cumulative over the lifetime of the lens and can lead to cataracts, the principal cause of blindness worldwide. In this work we study hypericin photosensitization of α-crystallin and detect extensive polymerization of bovine α-crystallin exposed in vitro to hypericin and UVA. We use fluorescence confocal microscopy to visualize binding between hypericin and α-crystallin in a human lens epithelial (HLE) cell line. Further, we show that UVA irradiation of hypericin-treated HLE cells results in a dramatic decrease in α-crystallin detection concurrent with a dramatic accumulation of the tryptophan oxidation product N-formylkynurenine (NFK). Examination of actin in HLE cells indicates that this cytoskeleton protein accumulates NFK resulting from hypericin-mediated photosensitization. This work also shows that filtration of wavelengths <400nm provides incomplete protection against α-crystallin modification and NFK accumulation, suggesting that even by wearing UV-blocking sunglasses, routine users of St. John's wort cannot adequately shield their lenses from hypericin-mediated photosensitized damage.

Christopher Foote's discovery of the role of singlet oxygen [1O2 (1Delta g)] in photosensitized oxidation reactions

The chemistry of singlet molecular oxygen [1O2 (1Delta g)], its importance in atmospheric, biological, and therapeutic processes, and its use as a reagent in organic synthesis have been of considerable interest. Many aspects of singlet oxygen chemistry have emanated from the work of Christopher S. Foote and co-workers. Singlet oxygen is a historically interesting molecule with an unusual story connected with its discovery. Foote and Wexler conducted experiments in the 1960s where evidence was obtained supporting 1O2 generation via two independent routes: (1) a photochemical reaction (dye-sensitized photooxidation) and (2) a chemical reaction (NaOCl with H2O2). An important factor in the discovery of 1O2 as the critical reaction intermediate in dye-sensitized photooxygenations was Foote's reassessment of the chemical literature of the 1930s, when 1O2 was suggested to be a viable intermediate in dye-sensitized photooxidation reactions. Experiments that used silica gel beads provided evidence for a volatile diffusible oxidant such as 1O2. However, a contemporaneous quarrel surrounded this early work, and the possible existence of solution-phase 1O2 was ignored for over 2 decades. Not long after Foote's initial studies were published in 1964, the idea of singlet oxygen as an intermediate in photooxidation chemistry gained increasing recognition and verification in organic, gas phase, and biological processes. There are many documented impacts that 1O2 has had and continues to have on biology and medicine, for example, photodynamic therapy and plant defenses.

Toxicity and drug interactions associated with herbal products: ephedra and St. John's Wort

Health care providers are being increasingly confronted with the use of herbal medications by their patients. It is imperative that patients be questioned regarding herbal preparation use and that health care providers become familiar with these agents. Research into the active components and mechanisms of action of various herbals is ongoing [350]. Long-range studies need to be performed to follow patients for efficacy or toxicity in chronic use [351,352]. Adverse reactions to herbal remedies should be reported to the FDA MedWatch at http://www.fda.gov/medwatch. As withany therapeutic agent, risk of use must always be weighed against potential benefits.