The role of lysines in Euglena cytochrome c-552. Chemical modification studies.

2-Hydroxy(p-hydroxyphenyl)-acetaldoxime, the alternative precursor to p-hydroxyphenylacetonitrile in dhurrin biosynthesis, was synthesized and its effectiveness as a substrate was examined in a microsomal enzyme system from sorghum seedlings. The hydroxyaldoxime was slowly converted to p-hydroxymandelonitrile when compared with p-hydroxyphenylacetonitrile and p-hydroxyphenylacetaldoxime. Moreover, radioactivity from U-¹⁴C]tyrosine was efficiently incorporated by trapping experiments into both the nitrile and aldoxime, but not into the hydroxyaldoxime. The reaction products formed on hydroxylation of the nitrile by the microsomal enzyme were identified as p-hydroxybenzaldehyde, HCN, and H₂O. Under anaerobic conditions, the nitrile was produced from the aldoxime and accumulated without undergoing hydroxylation. These results establish p-hydroxyphenylacetonitrile and not 2-hydroxy(p-hydroxyphenyl)-acetaldoxime as the intermediate in the conversion of p-hydroxyphenylacetaldoxime to p-hydroxymandelonitrile in dhurrin biosynthesis.