Figure 3.

K_m determinations for maize β-UP substrates. The β-UP activity was determined by measuring the amount of ¹⁴CO₂ released from [5-¹⁴C]β-ureidopropionate (A) or racemic [6-¹⁴C]β-ureidoisobutyrate (B) at pH 7 over 30 min at varying substrate concentrations. ○, Data points used to fit the curves to simple Michaelis-Menten equations (K_m for β-ureidopropionate = 11 μm and for β-ureidoisobutyrate = 6 μm; the latter value is calculated based on a single enantiomer being processed). Inhibition of the reaction seen at high concentrations of β-ureidoisobutyrate may be caused by the non-hydrolyzed enantiomer of the racemic substrate. These points (●) were not used to generate the fitted curve.