L-tyrosine regulation and biosynthesis via arogenate dehydrogenase in suspension-cultured cells ofNicotiana silvestrisSpeg. et Comes
✍ Scribed by C. G. Gaines; G. S. Byng; R. J. Whitaker; R. A. Jensen
- Book ID
- 104753814
- Publisher
- Springer-Verlag
- Year
- 1982
- Tongue
- English
- Weight
- 826 KB
- Volume
- 156
- Category
- Article
- ISSN
- 0032-0935
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✦ Synopsis
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-arogenate to 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 pfluoro-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-heptuloson- ate 7-phosphate synthase (EC 4.1.2.15) was insensitive to L-phenylalanine or L-tyrosine. The overall Abbreviations. DAHP synthase=3-deoxy-D-arabino-heptulosonate 7-phosphate synthase; 6FT=6-fluoro-DL-tryptophan; M F T : m-fluoro-DL-tyrosine; OFP = o-fluoro-DL-phenylalanine; PFP =p-fluoro-DL-phenylalanine features of this system indicate that MFT should be a very effective analog mimick for selection of feedback-insensitive regulatory mutants in L-tyrosine biosynthesis.