Age-dependency of molecular diffusion in the human anterior lens capsule assessed using fluorescence recovery after photobleaching

Purpose: To quantify the partition coefficient and the diffusion coefficient of metal-carrier proteins in the human lens capsule as a function of age. Methods: Whole lenses from human donors were incubated overnight in a solution of fluorescently labeled transferrin, albumin, or ceruloplasmin. In the central plane of the capsule thickness, fluorescence recovery after photobleaching (FRAP) experiments were conducted to measure the diffusion of the protein within the lens capsule. The anterior portion of the lens was recorded before the FRAP experiments to locate the boundaries of the anterior lens capsule and to measure the partition coefficient of the labeled proteins. The partition coefficient (P), the time to half maximum recovery of the fluorescent intensity (tau(1/2)), and the diffusion coefficient (D) for each protein were analyzed as a function of donor age. Results: There was no statistically significant relationship between the half maximum recovery time or the diffusion coefficient and age for transferrin (molecular weight [MW]=79.5 kDa, tau(1/2) =17.26 +/- 4.840 s, D=0.17 +/- 0.05 mu m(2)/s), serum albumin (MW=66.5 kDa, tau(1/2) = 18.45 +/- 6.110 s, D=0.17 +/- 0.06 mu m(2)/s), or ceruloplasmin (MW=120 kDa, tau(1/2) =36.57 +/- 5.660 s, D=0.08 +/- 0.01 mu m(2)/s). As expected, the larger protein (ceruloplasmin) took longer to recover fluorescent intensity due to its slower movement within the lens capsule. The partition coefficient statistically significantly increased with age for each protein (P-albumin: 0.09-0.71, P-ceruloplasmin: 0.42-0.95, P-transferrin: 0.19-1.17). Conclusions: The diffusion of heavy-metal protein carriers within the anterior lens capsule is not dependent on age, but it is dependent on the size of the protein. The permeability of the lens capsule to these heavy-metal protein carriers increases with age, suggesting that there will be a higher concentration of heavy metals in the older lens. This behavior may favor the formation of cataract, because heavy metals enhance protein oxidation through the Fenton reaction.