title: The in vitro biosynthesis of taxiphyllin and the channeling of intermediates in Triglochin maritima.
abstract: The in vitro biosynthesis of the cyanogenic glucoside taxiphyllin has recently been demonstrated in Triglochin maritima (Hösel, W., and Nahrstedt, A. (1980) Arch. Biochem. Biophys. 203, 753-757). We have now studied in more detail the multistep conversion of tyrosine into p-hydroxymandelonitrile, the immediate precursor of taxiphyllin, catalyzed by microsomes isolated from dark-grown seedlings. The biosynthetic pathway involves N-hydroxytyrosine, p-hydroxyphenylacetaldoxime, and p-hydroxyphenylacetonitrile. In marked contrast to an analogous pathway in Sorghum bicolor, p-hydroxyphenylacetonitrile is the best substrate for cyanide production (Vmax = 224 nmol/h/g, fresh wt) and the physiological substrate tyrosine is the poorest (Vmax = 18.8 nmol/h/g, fresh wt). The substrates exhibit alkaline pH optima between 7.5 and 9, and all except tyrosine show pronounced substrate inhibition. We have found that p-hydroxyphenylacetonitrile generated in situ from tyrosine is free to equilibrate by diffusion with exogenous material. On the other hand, neither N-hydroxytyrosine nor p-hydroxyphenylacetaldoxime will readily exchange with exogenous intermediates. We consider both N-hydroxytyrosine and p-hydroxyphenylacetaldoxime to be channeled in T. maritima, whereas in S. bicolor N-hydroxytyrosine and p-hydroxyphenylacetonitrile are channeled and the aldoxime is freely exchangeable.
mesh_terms: D002250:Carbon Radioisotopes; D003486:Cyanides / Q000378:metabolism; D004229:Dithiothreitol / Q000494:pharmacology; D005960:Glucosides / Q000096:biosynthesis*; D006027:Glycosides / Q000096:biosynthesis*; D006898:Hydroxylamines / Q000378:metabolism; D007700:Kinetics; D008861:Microsomes / Q000187:drug effects / Q000378:metabolism; D009570:Nitriles / Q000096:biosynthesis*; D010944:Plants / Q000378:metabolism*; D011865:Radioisotope Dilution Technique; D014316:Tritium; D014443:Tyrosine / Q000031:analogs & derivatives / Q000378:metabolism
chemical_list: D002250:Carbon Radioisotopes; D003486:Cyanides; D005960:Glucosides; D006027:Glycosides; D006898:Hydroxylamines; D009570:Nitriles; D014316:Tritium; C011360:taxiphyllin; D014443:Tyrosine; C020912:N-hydroxytyrosine; D004229:Dithiothreitol