The in vitro biosynthesis of taxiphyllin and the channeling of intermediates in Triglochin maritima.

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.