Multiple Forms of Plant Cytochromes P-450

Accumulating evidence indicates that there is a multiplicity of cytochrome P-450 enzymes in plants. These monooxygenases are implicated in the metabolism of sterols, terpenes, gibberellins, isoflavonoids, and xenobiotics. Evidence that cytochromes P-450 are involved in the detoxification of herbicides (chlorotoluron, primsulfuron, and diclofop) includes photoreversible CO inhibition of the reactions, and a requirement for O(2) and NADPH. Several cytochromes P-450, M(r) 45,000 to 65,000, have been isolated, including hydroxylases of cinnamic acid, 3,9-dihydroxypterocarpan, and digitoxin. In some cases the purified cytochrome P-450 has been successfully reconstituted with NADPH:cytochrome P-450 reductase (M(r) 72,000-84,000 protein). This reductase appears to be a nonspecific electron donor to different forms of cytochrome P-450. Immunological techniques and specific inhibitors (triazoles, imidazole derivatives) are being used to characterize plant cytochromes P-450 and the NADPH:cytochrome P-450 reductase. Specific cytochromes P-450 are induced by wounding or pathogens, others are expressed in specific cell types. Plant cytochromes P-450 are found in various subcellular locations, including endoplasmic reticulum, plasma membranes, glyoxysomes, and perhaps mitochondria. A cytochrome P-450 demethylase from avocado has recently been sequenced and found to have a hydrophobic N terminus similar to the membrane anchor of cytochromes P-450 from other organisms. The existence of cytochromes P-450 in different subcellular locations suggests that there are many genes for cytochromes P-450 in plants which have yet to be identified and classified.