The samples were incubated at 40 °C for 2 h at 900 rpm in a thermomixer (Thermomixer comfort, Eppendorf, Wesseling, Germany). The reaction was stopped by adding 20 μL 2 m trichloroacetic acid (Roth). After centrifugation for 15 min at 9000 × g, 150 μL of the supernatant were mixed with 50 μL 1 m NaOH (Roth). Absorption at 450 nm was measured against a blank in a micro plate reader (Sunrise Remote, Tecan, Crailsheim, Germany).