Role of Cytochrome P450 in Tulip Bulb Probed by Substrate‐Binding Studies

Microsomal fractions from dormant and wounded (sliced, aged) tulip bulbs showed a Soret peak at 448 nm in the carbon monoxide difference spectrum of dithionite-treated membranes characteristic of cytochrome P450. Wounding and ageing induced a seven-fold increase in P450 content. The enzyme-substrate binding spectra and substrate affinity constants found under different conditions implied different forms of cytochrome P450. Of the several xenobiotics examined, two of them, benzo[a]pyrene and the herbicide 2,4-dichlorophenoxyacetic acid, were inferred to be substrates from the production of P450 substrate-binding spectra and varied substrate affinity constants were observed using the different microsomal preparations. In the wounded tissue, geraniol binding was also observed, which may reflect the endogenous function of the P450, possibly as part of a defence mechanism.