QUANTITATION OF ULTRAVIOLET LIGHT-ABSORBING FRACTIONS OF THE CORNEA

This study attempts to measure the quantitative contribution of major chemical fractions of the whole bovine cornea to ultraviolet (UV) absorption between 240 and 300 nm, with special attention to the biologically significant range of 290-300 nm. The cornea was divided into water-insoluble, nonprotein small water-soluble, water-soluble protein, and lipid-soluble fractions. The insoluble fraction (largely collagen) was solubilized by enzymatic digestion. Solutions of individual fractions equivalent to a constant mass of fresh cornea were scanned for absorption from 240 to 300 nm. The sum of the absorbances of the individual fractions closely approximated the absorbance of whole corneas at all wavelengths examined. The extinction coefficients of the lipid and water-soluble fractions were several times greater than that of the insoluble fraction throughout the studied spectrum. Yet, because of its large mass (75% of cornea dry weight), the insoluble fraction accounted for 40-50% of UV absorbance between 240 and 280 nm. However, in the range of 290-300 nm, the water-soluble plus lipid-soluble fractions accounted for 60-65% of the total absorption, with the water-soluble proteins alone accounting for 40-45% of the total. The soluble proteins comprised only similar to 17% of the cornea's dry weight. The special contribution of the water-soluble proteins to absorption was attributed to their relatively high tryptophan content (similar to 1.6% by weight). A 54-kDa protein, identified by others as tryptophan-rich, tumor-associated aldehyde dehydrogenase, accounted for similar to 30% of the total soluble protein mass. The 54-kDa soluble polypeptide together with other water-soluble proteins made the dominant contribution to the total UV-absorbing capacity of the cornea in the 290-300 nm range. We suggest the name absorbins for these proteins.