S-Reticuline

Fig. 10.4 Reaction catalyzed by the cytochrome P-450-dependent monooxygenase (5)-7V-methylcoclaurine 3 -hydroxylase (CYP80B1) along the (

In order to understand the continuation of the biosynthesis of codeine and morphine from reticuline, the structure for (S)-reticuline can be written as follows ... 

Figure 5 Two different formulas showing the stereochemistry of (S)-reticuline...

The majority of recent biosynthetic studies on bisbenzylisoquinoline alkaloids has been done by Bhakuni and co-workers, using feeding of isotopically labeled precursors to alkaloid-producing plants this work has been reviewed (339,340). In this manner, several of these alkaloids were shown to be derived by coupling of coclaurine or V-methylcoclaurine units the thalicarpine family of alkaloids, however, is formed from two (S)-reticuline (430a) units (148,339). 

Fig. 17 Sequential enzymatic formation of berberine from (S)-reticuline (BBE berberine bridge enzyme SOMT scoulerine O-methyltransferase THCOX tetrahydrocolumbamine oxidase COX columbamine oxidase)...

Ounaroon, A., G. Decker, J. Schmidt, F. Lottspeich and T.M. Kutchan, (R,S)-Reticuline 7-O-methyltransferase and (R,S)-Norcoclaurine 6-O-Methyltransferase of Papaver... 

It is known that berberine (46) and the protoberberine alkaloid (S)-stylopine (44) are elaborated from an intact molecule of (S)-reticuline [as (33)], the 7V-methyl group providing C-8 in each alkaloid1,2,42 (cf. Vol. 7, p. 12). Exactly similar findings were obtained for tetrahydropalmatine (45) and palmatine (47) in C. laurifolius.43 Of several benzylisoquinolines tested, only reticuline (33), nor-reticuline (41), and norlaudanosoline (40) were significantly incorporated. Tetrahydropalmatine (45) was found to be converted into palmatine (47) and the conversion was irreversible. 

S)-Reticuline is a branch-point intermediate in the biosynthesis of most BAs. Most work has focused on branch pathways leading to the benzophenanthridine (e.g., sanguinarine), protoberberine (e.g., berberine), and morphinan (e.g., morphine and codeine) alkaloids.19 Most enzymes involved have been isolated, many have been purified, and four corresponding cDNAs have been cloned.19 The first committed step in benzophenanthridine and protoberberine alkaloid biosynthesis involves the conversion of (S)-reticuline to (5)-scoulerine by the berberine bridge enzyme (BBE) (Fig.7.2). BBE was purified from Berberis beaniana,20 corresponding cDNAs were cloned from E. californica and B. stolonifera,21 22 and BBE genes have been isolated from P. somniferum and E. californica.23,24... 

Conversion of (S)-reticuline to its ( )-epimer is the first committed step in morphinan alkaloid biosynthesis in certain species. 1,2-Dehydroreticuline reductase catalyzes the stereospecific reduction of 1,2-dehydroreticuline to (7 )-reticuline.39 Intramolecular carbon-carbon phenol coupling of (if)-reticuline by the P450-dependent enzyme salutaridine synthase (STS) results in the formation of salutaridine.40 The cytosolic enzyme, salutaridine NADPH 7-oxidoreductase (SOR), found in Papaver bracteatum and P. somniferum, reduces salutaridine to (7S)-salutaridinol.41 Conversion of (7S)-salutaridinol into thebaine requires closure of an oxide bridge between C-4 and C-5 by acetyl coenzyme A salutaridinol-7-0-acetyltransferase (SAT). The enzyme was purified from opium poppy cultures and the corresponding gene recently isolated (Fig.7.2).42,43 In the last steps of morphine... 

FRENZEL, T., ZENK, M.H., S-Adenosyl-L-methionine 3 -hydroxy-N-methyl-(stereoselective enzyme of the (S)-reticuline pathway. Phytochemistry, 1990,29,3505-3511. 

Conversion of reticuline to salutaridine by rat liver, the critical step that generates the morphine skeleton, was later realized (263), and ambiguities clarified (271). This firmly established that mammalian liver is capable of converting (/ )-reticuline to (-i-)-salutaridine in a coupling reaction which occurs with the help of a microsomal cytochrome P-450 enzyme. The reaction was found to be NADPH dependent and enantiospecific since it did not proceed with (S)-reticuline. The pH optimum was found to be 7.8, and omission of oxygen or NADPH rendered the enzyme complex inactive. Typical inhibitors of P-450 enzymes, as well as exposure to carbon monoxide, halted the formation of (-l-)-salutaridine. These findings, together with a report that intravenous administration of (-l-)salutaridine. 

S)-reticuline to ( )-reticuline in Papver somniferum is achieved by enantiospecific oxidation of the (5 )-enantiomer (273), and it is followed by an NADPH-dependent enzymatic reduction of the dehydroreticulinium ion, which also is highly substrate specific since no reduction of 1,2-dehydronorreticuline was observed (275). 

S)-Reticuline is readily formed from (S)-norcoclaurine as a result of a series of hydroxylations and methylations. From intermediates observed in vivo and enzyme studies, it may be concluded that (S)-norcoclaurine is... 

S)-Reticuline is the precursor for aporphine alkaloids (Fig. 2.1). Minami et al (2008) succeeded fo express fhe genes of magnoflorine bios)mthesis (NCS, CNMT, 40MT, CYP80G2) to produce corytuberine and magnoflorine in recombinant E. coli and S. cerevisiae. 

S)-Reticuline is also the precursor for the biosynthesis of benzophenanthri-dine (e.g. sanguinarine, marcarpine), protoberberine, berberine, palmatine) and morphinan alkaloids (morphine, codeine) (see next few paragraphs). 

Figure 2.8 Biosynthesis of morphine via the conversion of (S)-reticuline to (R)-reticuline, salutaridine and thebaine. SalS, salutaridine synthase SaIR, salutaridine reductase THS, thebaine synthase COR, codeinone reductase.

The role of reticuline as an intermediate in the biosynthesis of the mor-phinan alkaloids (Fig. 2.8) was demonstrated by the isolation both of (S)-and (f )-reticuline from the opium poppy. An excess of the (S)-reticuline over the (f )-isomer was found in opium (poppy latex) obtained from the mature plant, in contrast to the roughly equal amounts of these two isomers that occur in poppy seedlings. Both isomers were found to be incorporated into morphine, the major alkaloid isolated from opium, although incorporation of the (f )-isomer was slightly more efficient. (f )-Reticuline is firmly established in P. somniferum as the precursor of the morphinan-type alkaloids (Loefer and Zenk, 1990). (S)-Reticuline, however, is the central intermediate in isoquinoline alkaloid biosynthesis. It has been postulated that (R)-reticuline is formed from (S)-reticuline by isomerization. This inversion of configuration can be explained by the intermediate formation of the 1,2-dehydroreticulinium ion originating from (S)-reticuline, followed by stereospecific reduction to yield the (R) counterpart. The 1,2-dehydroreticulinium ion is efficiently incorporated into opium alkaloids and its role as a precursor of the morphinan-t)q)e alkaloids has been unequivocally established (De-Eknamkul and Zenk, 1990, 1992). 

The conversion of (S)-reticuline to 1,2-dehydroreticuline has been accomplished using a novel oxidase isolated from cell cultures of plants of the... 

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