Functional Transport Properties of Human Zinc Transporter 1: Kinetics and pH-Dependency

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Abstract
Intracellular zinc (Zn2+) homeostasis is essential for physiological and pathological processes and is strictly regulated by Zn2+ transporters. Zinc transporter 1 (ZnT1) is a ubiquitously expressed plasma membrane-localized Zn transporter that exports Zn2+ 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 Zn2+ with high affinity in pH-dependent and pH-independent manners. The Km and Vmax of pH-dependent Zn2+ transport were 0.40 μM and 15.13 nmol/min/mg protein, and those of pH-independent Zn2+ transport were 0.52 μM and 8.88 nmol/min/mg protein (low concentrations of Zn2+), 3.02 μM and 17.59 nmol/min/mg protein (high concentrations of Zn2+), respectively, suggesting biphasic kinetic components of Zn2+ transport. Even without pH gradient formation, ZnT1 exhibits potent Zn2+ transport activity. In pH dependency, Zn2+ 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 Zn2+ transport activity decreased at an outside pH of 8.0, despite an increase in ΔpH. Although previous studies have proposed that ZnT1-mediated Zn2+ transport activity is driven by a calcium (Ca2+) gradient and not by a pH gradient, Ca2+ does not enhance Zn2+ transport activity in the presence or absence of a pH gradient. These results strongly suggest that ZnT1 protein transports Zn2+ optimally at a specific pH and exports excess intracellular Zn2+ even without ΔpH.

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