The biosynthesis of cyanogenic glucosides in higher plants. Channeling of intermediates in dhurrin biosynthesis by a microsomal system from Sorghum bicolor (linn) Moench.
The biosynthetic pathway for the cyanogenic glucoside, dhurrin, involves the following intermediates: L-tyrosine, N-hydroxytyrosine, p-hydroxyphenylacetaldoxime, p-hydroxyphenylacetonitrile, and p-hydroxymandelonitrile. N-Hydroxytyrosine and p-hydroxy-phenylacetonitrile produced from L-tyrosine by microsomes from seedlings of Sorghum bicolor are utilized more effectively as substrates than exogenously added N-hydroxytyrosine and p-hydroxyphenylacetonitrile. The minimum values for the channeling ratios are 25 for N-hydroxytyrosine and 115 for p-hydroxyphenylacetonitrile. On the other hand, p-hydroxyphenylacetaldoxime produced internally exchanges readily with exogenously added p-hydroxyphenylacetaldoxime. These results indicate that the biosynthetic pathway is catalyzed by two mutienzyme complexes or by two multifunctional proteins and explain why the rate of the overall sequential reaction starting from L-tyrosine is greater than the rates of reaction initiated later in the sequence with the known intermediates N-hydroxytyrosine and p-hydroxyphenylacetonitrile. Attempts to cross-link chemically the last enzyme in the pathway, a soluble UDP-glucose glucosyl-transferase, to the microsomal system were unsuccesful.
D002250:Carbon Radioisotopes; D005960:Glucosides / Q000096:biosynthesis*; D006027:Glycosides / Q000096:biosynthesis*; D007553:Isotope Labeling; D007700:Kinetics; D008861:Microsomes / Q000378:metabolism*; D009570:Nitriles / Q000096:biosynthesis* / Q000378:metabolism; D010091:Oximes / Q000378:metabolism; D010944:Plants / Q000378:metabolism*; D014316:Tritium; D014443:Tyrosine / Q000378:metabolism
D002250:Carbon Radioisotopes; D005960:Glucosides; D006027:Glycosides; D009570:Nitriles; D010091:Oximes; D014316:Tritium; C020363:4-hydroxyphenylacetaldoxime; D014443:Tyrosine; C011220:dhurrin