Quantitative distribution of Zn, Fe and Cu in the human lens and study of the Zn–metallothionein redox system in cultured lens epithelial cells by elemental MS

Abstract

The human lens is constantly subjected to exogenous and endogenous stressors, leading to oxidative cellular damage and, with time, to cataract formation. Metallothioneins (MTs) are a group of important enzymes that use metals (i.e., Zn and Cu) to protect tissues from the deleterious effects of free radicals associated with oxidative stress. This work combines elemental mass spectrometry with bio-analytical methodologies to determine (i) the total amount, the quantitative speciation and the cellular distribution of trace elements (i.e., Zn, Fe, and Cu) in human lenses and their corresponding capsules; and (ii) the effects of “exogenous” metal (i.e., ⁶⁸ZnSO₄, isotopically enriched in ⁶⁸Zn) and stressor (i.e., IL-1α) on the zinc–MT redox system in cultured human lens epithelial cells (HLEsv) in vitro. Of all the elements analyzed, Zn was the most abundant, and it was equally present in both the capsule (9.7 ± 2.5 μg g⁻¹ tissue) and the lens (9.5 ± 1.2 μg g⁻¹ tissue). In contrast, Fe was found to be more than 6-fold more abundant in the capsule (1.6 ± 0.4 μg g⁻¹) than in the lens (0.2 ± 0.1 μg g⁻¹). Zinc in the lens is mainly associated with high molecular mass proteins, whereas in the capsule it is mostly bound to low and medium molecular mass proteins. The localization of Zn, Cu and MTs in the lens showed their preferential co-distribution in the lens epithelial cell layer, underneath the anterior capsule. Exogenous Zn is capable of inducing a stoichiometric change in MT proteins from Zn₃–MT to Zn₇–MT within lens epithelial cells in vitro, which may be related to their antioxidant capacity.