Paramagnetic species in cataractous human lenses

Abstract

KURZEL et al.¹ studied the low temperature phosphorescence emission from normal and cataractous human lenses at 77 K. After comparing their lens data with those obtained from tryptophan in a frozen ethanol–water glass, they concluded that the phosphorescence of the lens is caused by the tryptophan residues of lens proteins. This was based on their finding in the lens a phosphorescence λ(max), phosophorescence excitation and phosphorescence quantum yield consistent with tryptophan and differing significantly from those of phenylalanine and tyrosine. Here we present the results of an electron spin resonance (ESR) study which characterises the paramagnetic triplet state of normal and cataractous human lenses and provides proof that it stems from the tryptophan moiety and not tyrosine or phenylalanine. This supplements previous observations that tryptophan is the dominant species in cataractous human lenses, giving rise to phosphorescent emission. In addition, we have discovered that the lens excited triplet (phosphorescent) tryptophan is associated with the production of a free radical; we believe that this free radical may be the missing link between ultraviolet irradiation and lens photodamage leading to cataract formation^3,4.