Tyrosine biosynthesis in Sorghum bicolor: isolation and regulatory properties of arogenate dehydrogenase

The conversion of prephenic acid to tyrosine can occur by two different routes: (a) oxidative decarboxylation (prephenate dehydrogenase) followed by transamination (aromatic aminotransferase); (b) transamination of prephenate forming the non-aromatic amino acid arogenic acid (prephenate aminotransferase) followed by oxidative decarboxylation (arogenate dehydrogenase). High activity of arogenate dehydrogenase was found in extracts of etiolated sorghum seedlings, while no evidence of prephenate dehydrogenase was observed. Arogenate dehydrogenase from sorghum eluted, with high recovery of activity (93%), as a single peak on DEAE-cellulose chromatography. The enzyme was strongly inhibited by tyrosine but was unaffected by phenylalanine, prephenate, or tryptophan. Kinetic analysis showed that tyrosine inhibition was competitive with arogenate and that the Ki for tyrosine (61 microM) was much smaller than the Km for arogenate (350 microM). The properties of arogenate dehydrogenase indicate that this enzyme is important in the regulation of tyrosine biosynthesis in sorghum. Strong inhibition of the enzyme by tyrosine may indicate that arogenate is a branch point in the shikimate pathway in plants and therefore arogenate may be a precursor to phenylalanine and the numerous phenylpropanoid secondary metabolites derived from phenylalanine.