Cytochrome Components of Plant Microsomes

Abstract

The electron transport components of the microsomal fraction of cauliflower buds and mung bean hypocotyls were investigated using split-beam and dual wavelength spectrophotometry under a variety of reducing conditions. Cauliflower microsomes were found to contain an ascorbate-reducible component, termed cytochrome b-559.5 [E′₀=+ 135 ± 20 mV; λ_max (reduced minus oxidised) = 559.5, 527 and 429 nm at 23 °C], cytochrome b₅ [E'₀=−20 ± 20 mV; λ_max (reduced minus oxidised) = 556, 526 and 425 nm at 23°C], cytochromes P-450 and P-420. On the basis of binding studies with ethyl isocyanide, degradation of cytochrome P-450 to P-420, redox potential, aniline binding, and relative rates of reduction by NADPH and NADH, it is suggested that the cytochrome P-450 system is analogous to that in mammalian microsomes. Other components, reducible only by dithionite, may also be present. Mung bean microsomes were found to contain an ascorbate-reducible component, termed cytochrome b-562 [E′₀=+ 120 ± 20 mV; λ_max (reduced minus oxidised) = 562, 528, and 430 nm at 23 °C], cytochrome b₅, and a low potential component which was reducible only by sodium dithionite. No cytochrome P-450 or P-420 could be detected. A general method of analysis of the cytochromes was developed and applied to the microsomes from a variety of plant sources. The results indicate that large variations, both in type and amount of components, occur between the microsomes from different plant materials.