Fluorescence quenching studies of the structures of calf gamma-II, III, and IV crystallins.

The structures of the calf lens crystallin fractions gamma-II, gamma-III, and gamma-IV have been investigated using the fluorescence quenching method. The three crystallin fractions showed very large differences in the quenching rates of their fluorescent tryptophan residues, for quenching by acrylamide or iodide in pH 7.5 phosphate buffer solutions. The experimentally measured quenching rate constants were kq(II) = 3.2 x 10(8), kq(III) = 9.9 x 10(8), and kq(IV) = 1.8 x 10(9) M-1 sec-1. Smaller rate constants were obtained for iodide quenching of the three crystallins, but the values were in approximately the same ratios as the ones found for acrylamide quenching. The conclusion is that the tryptophan residues in gamma-II crystallin are 6-10 times less easily quenched than those of gamma-IV crystallin and 3-6 times less easily quenched than those of gamma-III. These conclusions are in accord with those reached by Mandal et. al. based on fluorescence and CD data, who found the following order of Trp hydrophobicities: gamma-II greater than gamma-III greater than gamma-IV. The significance of these structural differences for lens function and stability remains to be elucidated.