title: L-tyrosine regulation and biosynthesis via arogenate dehydrogenase in suspension-cultured cells of Nicotiana silvestris Speg. et Comes.
abstract: The biosynthetic route to L-tyrosine was identified in isogenic suspension-cultured cells of N. silvestris. Arogenate (NADP(+)) dehydrogenase, the essential enzyme responsible for the conversion of L-arogenato L-tyrosine, was readily observed in crude extracts. In contrast, prephenate dehydrogenase (EC 1.3.1.13) activity with either NAD(+) or NADP(+) was absent altogether. Therefore, it seems likely that this tobacco species utilizes the arogenate pathway as the exclusive metabolic route to L-tyrosine. L-Tyrosine (but not L-phenylalanine) was a very effective endproduct inhibitor of arogenate dehydrogenase. In addition, analogs of L-tyrosine (m-fluoro-DL-tyrosine [MFT], D-tyrosine and N-acetyl-DL-tyrosine), but not of L-phenylalanine (o-fluoro-DL-phenylalanine and p-fluoro-DL-phenylalanine), were able to cause inhibition of arogenate dehydrogenase. The potent antimetabolite of L-tryptophan, 6-fluoro-DL-tryptophan, had no effect upon arogenate dehydrogenase activity. Of the compounds tested, MFT was actually more effective as an inhibitor of arogenate dehydrogenase than was L-tyrosine. Since MFT was found to be a potent antimetabolite inhibitor of growth in N. silvestris and since inhibition was specifically and effectively reversed by L-tyrosine, arogenate dehydrogenase is an outstanding candidate as the in vivo target of analog action. Although chorismate mutase (EC 5.4.99.5) cannot be the prime target of MFT action, MFT can mimick L-tyrosine in partially inhibiting this enzyme activity. The activity of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase (EC 4.1.2.15) was insensitive to L-phenylalanine or L-tyrosine. The overall features of this system indicate that MFT should be a very effective analog mimick for selection of feedback-insensitive regulatory mutants L-tyrosine biosynthesis.
authors.affiliation: Center for Somatic-cell Genetics and Biochemistry, Department of Biological Sciences, State University of New York, 13901, Binghamton, NY, USA.
authors.forename: C G
authors.initials: CG
authors.lastname: Gaines
authors.forename: G S
authors.initials: GS
authors.lastname: Byng
authors.forename: R J
authors.initials: RJ
authors.lastname: Whitaker
authors.forename: R A
authors.initials: RA
authors.lastname: Jensen