Abstract
The present investigations were primarily aimed at confirming the presence and defining the properties and function of cytochrome P-450 in higher plants. This involved a survey of the metabolism of various compounds including, fatty acid oxidation, the biosynthesis of phenolic compounds, as well as 'foreign compounds'. Comparative studies were undertaken with hepatic and extra-hepatic mammalian tissues. Cytochrome P-450 was demonstrated in both microsomal and mitochondrial fractions from various mammalian tissues; but is associated only with the 105,000g x 60 min pellet fractions of the higher plants investigated, including avocado pear, pea seedlings, bean cotyledons and sweet com seedlings. With the exception of the avocado mesocarp, the cytochrome P-450 detected in higher plants was found to be transient in the absence of added thiol reagents and glycerol. The avocado pear type of 'P-450' was more akin to rat liver cytochrome P-450 especially in response to storage, deoxycholate and phospholipase C treatment. Binding spectra studies with various endogenous and 'foreign' substrates metabolized by the liver microsomal mixed function oxidase system showed that the capacity of plant tissue is much more limited. A variety of compounds elevated hepatic microsomal cytochrome P-450, but only safrole significantly altered levels in mesocarp tissues, while in other plant tissues,levels remained unaltered. The ability of some plant tissues (especially avocado mesocarp) to metabolize a variety of anutrients including the hydroxylation of aniline, benzo[a]pyrene and biphenyl, N-demethylation of p-chloro-N-methyl-aniline and the 0-demethylation of p-nitroanisole were compared with rat liver. These activities were either not detectable in plant material or were present at one or more orders of magnitude lower than those found in hepatic tissues. Levels found in extra-hepatic mammalian tissues seemed more akin both in terms of quality and quantity with those found in avocardo mesocarp. Both NADP[2]-cytochrome c and NADPH[2]-cytochrome P-450 reductases were detected in hepatic and mesocarp 105,000g fractions. Inhibitors effective in mammalian cytochrome P-450 mediated reactions caused little or no effect when tested with corresponding reactions in higher plants, this may be a problem of penetration rather than binding. NADH[2] was capable of replacing NADPH[2] in plant reductase systems. Mesocarp and hepatic fractions were found to metabolize lauric acid, and although identifications of metabolites has not been completed, inhibition studies and cofactor requirements indicate possible cytochrome P-450 mediation. Similar studies on cinnamic acid hydroxylation indicate a role for the cytochrome in the biosynthesis of phenolics in various higher plants.