Kinetic and regulatory properties of arogenate dehydratase in seedlings of Sorghum bicolor (L.) Moench.

Arogenate dehydratase was purified sixfold from an extract of etiolated seedlings of Sorghum bicolor. Prephenate dehydratase was not detected. The arogenate dehydratase activity displayed hyperbolic substrate kinetics with a KM for arogenate of 0.32 mM. Activity was inhibited competitively by phenylalanine and was stimulated by tyrosine. The low KI for phenylalanine (24 microM) and KA for tyrosine (2.5 microM) indicated a high affinity of the enzyme for these effectors. These results establish the routing of metabolites in phenylalanine biosynthesis in sorghum as proceeding via arogenate rather than phenylpyruvate.