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.