Cytochrome pretreated with promoter

The above sequence mimics the proposed biosynthesis of Ervatamia alkaloids and in this context Thai and Mansuy (190) set out to determine whether an enzyme preparation would be able to promote the same transformation. By incubation of dregamine hydrochloride with a suspension of liver microsomes from a rat pretreated with phenobarbital (as a good inducer of P-450 cytochromes) in the presence of NADPH and 02, 20-epiervatamine (45) was formed together with the major metabolite Nl -demethyldregamine. It is well known that microsomal reaction on tertiary amines results in Af-oxide formation or N-deal-kylation. Thus it is likely that 45 was derived either from a rearrangement of dregamine JV4-oxide, catalyzed by the iron cytochrome P-450 or from one-electron oxidation of 30. 

An important drug in the present context is the mineralocorticoid receptor antagonist spironolactone (7.74, Fig. 7.12). Among its many metabolic reactions, spironolactone is readily hydrolyzed at the thioester bond (Fig. 7.12, Reaction a) to form deacetyl-spironolactone (7.75, Fig. 7.12), a metabolite found in a variety of tissues [155 -157]. This thiol compound, which is also a potent mineralocorticoid antagonist, promotes the mechanism-based inactivation of hepatic, adrenal, and testicular cytochrome P450 isozymes. There is now good evidence to indicate that this behavior is the result of microsomal 5-oxidation (see Chapt. 7 in [7]). When spironolactone was incubated with liver microsomes from rats pretreated with dexamethasone (an inducer of CYP3A), the sulfinic and sulfonic acid derivatives were characterized [158]. Perhaps the importance of the 5-deacetylation of spironolactone... 

Progress has been made in demonstrating synthesis of specific proteins by cell-free systems, e.g., on the synthesis of cytochrome c by isolated rat liver mitochondria (S3, cf. 383) and on the synthesis of serum albumin by the isolated microsome fraction of rat liver 34, cf. 335,336). Campbell et al. 34) concluded that while specific serum albumin is, indeed, synthesized on the ribonucleoprotein particle fraction of the microsomes, it is not readily released in soluble form. In other words, the isolated microsomes have lost their ability to promote substrate turnover. The same is true for the hemoglobin-synthesizing RNP particles from rabbit reticulocytes 35, cf. 138). Ogata and associates 36) have essentially confirmed the results of Campbell et al. 34) on the synthesis of serum albumin by liver microsomes they have also studied the relative effect of both stimulatory and inhibitory factors on the incorporation of different amino acids into total ribonucleoprotein and into serum albumin, and showed that the requirements for the two processes were generally the same it may be noted, however, that pretreatment of the pH 5 enzymes with ribonuclease, which caused a 95% inhibition of the ineorporation into ribonucleoprotein, inhibited the corresponding incorporation into serum albumin by only 55%. 

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