Coffee contains hydroxyhydroquinone (HHQ). HHQ is one of by-products released during bean roasting. Therefore, it is important to elucidate the bioactivity of HHQ to predict its beneficial or adverse effects on humans. We studied zinc-dependent and independent actions of commercially-procured synthetic HHQ in rat thymocytes using flow cytometric techniques with propidium iodide, FluoZin-3-AM, 5-chloromethylfluorescein diacetate, and annexin V-FITC. HHQ at 1050 μM elevated intracellular Zn2+ levels by releasing intracellular Zn2+. HHQ at 10 μM increased cellular thiol content in a Zinc-dependent manner. However, HHQ at 30–50 μM reduced cellular thiol content. Although the latter actions of HHQ (30–50 μM) were suggested to increase cell vulnerability to oxidative stress, HHQ at 0.3–100 μM significantly protected cells against oxidative stress induced by H2O2. The process of cell death induced by H2O2 was delayed by HHQ, although both H2O2 and HHQ increased the population of annexin V-positive living cells. However, HHQ at 10–30 μM promoted cell death induced by A23187, a calcium ionophore. HHQ at 10–30 μM exerted contrasting effects on cell death caused by oxidative stress and Ca2+ overload. Because HHQ is considered to possess diverse cellular actions, coffee with reduced amount of HHQ may be preferable to avoid potential adverse effects
