Tyrp1tr diphenol oxidase activity measured to mimic pH conditions in the ER (pH 7.2) and melanosomes (pH 5.5). Panel A: Tyrp1tr diphenol oxidase activities measured at pH 7.2 (grey bars) and 5.5 (brown bars) using DHICA as a substrate. The reaction mixture containing 3 mM DHICA and 3 mM MBTH in GF buffer including 50 µM TCEP, pH 7.2 or 5.5, was incubated for 180 min at 4, 25, 31, 37, and 43 °C and monitored by measuring the IQCA/MBTH formation at 505 nm. Panel B: Tyrp1tr diphenol oxidase activities measured at pH 7.2 (grey bars) and 5.5 (brown bars) using DHICA as a substrate in the presence of different concentrations of TCEP. The reaction mixtures containing 3 mM DHICA, 3 mM MBTH, and 0, 125, or 500 µM TCEP in GF buffer, pH 7.2 and 5.5 were incubated for 180 min at 37 °C and monitored by measuring the IQCA/MBTH complex formation at 505 nm.