The 4-Hydroxylation of Cinnamic Acid by Sorghum Microsomes and the Requirement for Cytochrome P-450

Cinnamic acid 4-hydroxylase in etiolated sorghum seedlings was concentrated in the microsomal or light membrane fraction upon extraction. The three substrates required in the catalysis, cinnamic acid, TPNH, and O2 have Km values in the micromolar range. The same microsomal fraction contained cytochromes P-450 and b5 at 0.10 and 0.55 nmoles · mg of protein-1, respectively. Preparation of a photodissociation action spectrum on carbon monoxide inhibition of the cinnamic acid hydroxylase indicated a requirement for cytochrome P-450 in the catalysis. When challenged with a range of potential inhibitors, the hydroxylase behaved in a manner that is fairly typical of the more extensively studied P-450 monooxygenases of nonplant tissues.