Localization of Cinnamic Acid 4-Monooxygenase and the Membrane-bound Enzyme System for Dhurrin Biosynthesis in Sorghum Seedlings

Abstract

The localization of three monooxygenase (hydroxylase) enzyme systems which occur in dark-grown seedlings of Sorghum bicolor has been studied. Cinnamic acid 4-hydroxylase (CAH) (trans-cinnamate 4-monooxygenase, EC 1.14.13.11), which has been increasingly utilized in plants as a marker for the endoplasmic reticulum, migrated with that fraction in continuous and discontinuous sucrose gradients. When 10 mm MgCl₂ was used to shift the density banding of the marker enzyme, NADPH cytochrome c reductase, from 1.12 to 1.17 g/cm³, the CAH activity was displaced as well. The membrane-bound enzyme system involved in the biosynthesis of the cyanogenic glucoside dhurrin was also shown to be closely associated with the endoplasmic reticulum. This system contains hydroxylases capable of hydroxylating tyrosine to form N-hydroxytyrosine and hydroxylating p-hydroxyphenylacetonitrile to form p-hydroxy-(S)-mandelonitrile.