Pharmaceutical Society of Japan Acta Medica Okayama 0918-6158 49 2 2026 Functional Transport Properties of Human Zinc Transporter 1: Kinetics and pH-Dependency 364 370 EN Yuma Yoshioka Department of Molecular Membrane Biology, Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University Takaaki Miyaji Department of Molecular Membrane Biology, Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University 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. No potential conflict of interest relevant to this article was reported.