Intracellular zinc (Zn 2+ ) homeostasis is essential for physiological and pathological processes and is strictly regulated by Zn 2+ transporters. Zinc transporter 1 (ZnT1) is a ubiquitously expressed plasma membrane-localized Zn transporter that exports Zn 2+ from the cytoplasm to the extracellular space. However, the functional transport properties regarding kinetics and driving forces of ZnT1 remain debatable. In this study, we established a cell-free proteoliposome assay system and demonstrated that ZnT1 transports Zn 2+ with high affinity in pH-dependent and pH-independent manners. The K m and V max of pH-dependent Zn 2+ transport were 0.40 μ M and 15.13 nmol/min/mg protein, and those of pH-independent Zn 2+ transport were 0.52 μ M and 8.88 nmol/min/mg protein (low concentrations of Zn 2+ ), 3.02 μ M and 17.59 nmol/min/mg protein (high concentrations of Zn 2+ ), respectively, suggesting biphasic kinetic components of Zn 2+ transport. Even without pH gradient formation, ZnT1 exhibits potent Zn 2+ transport activity. In pH dependency, Zn 2+ transport activity was higher at an inside pH of 6.0 than at 6.5–7.5 for proteoliposomes, despite the same ΔpH of 0.5–1.5. The Zn 2+ transport activity decreased at an outside pH of 8.0, despite an increase in ΔpH. Although previous studies have proposed that ZnT1-mediated Zn 2+ transport activity is driven by a calcium (Ca 2+ ) gradient and not by a pH gradient, Ca 2+ does not enhance Zn 2+ transport activity in the presence or absence of a pH gradient. These results strongly suggest that ZnT1 protein transports Zn 2+ optimally at a specific pH and exports excess intracellular Zn 2+ even without ΔpH.
Yoshioka et al. (Fri,) studied this question.