1,2-Dehydroreticuline reductase

Scheme 29 The stereochemical inversion of (S)-(+)- to (R)-(-) reticuline via the enzymes (S)-reticulinium oxygenase and 1,2-dehydroreticuline reductase. The 1,2-dehydro-reticulinium ion is proposed as an intermediate.

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... 

DE-EKNAMKUL, W., ZENK, M.H., Purification and properties of 1,2-dehydroreticuline reductase from Papaver somniferum seedlings. Phytochemistry, 1992, 31,813-821. 

Scheme 1. Biosynthesis of the benzylisoquinoline alkaloids berberine, morphine, and sang-uinarine. Enzymes for which corresponding molecular clones have been isolated are shown in bold. Abbreviations 4 OMT, 3 -hydroxy-A-methylcoclaurine 4 -0-methyltransferase 60MT, norcoclaurine 6-0-methyltransferase 70MT, reticuline 7-0-methyltransferase BBE, berberine bridge enzyme CFS, cheilanthifoline synthase CNMT, coclaurine A-methyltransf-erase COR, codeinone reductase CYP719A1, canadine synthase CYP80A1, berbamunine synthase CYP80B1, A-methylcoclaurine 3 -hydroxylase DBOX, dihydrobenzophenanthri-dine oxidase DRR, 1,2-dehydroreticuline reductase DRS, 1,2-dehydro reticuline synthase MSH, A-methylstylopine 14-hydroxylase NCS, norcoclaurine synthase P6H,...

The later steps of morphine biosynthesis have been investigated in P. somniferum cells and tissue. Notably, in morphine biosynthesis, (S)-reticuline is converted to (R)-reticuline, thereby epimerizing the stereocenter generated by norcoclaurine synthase at the start of the pathway (Fig. Id). (S)-reticuline is converted to (R)-reticuline through a 1,2-dehydroreticuline intermediate. Dehydroreticuline synthase catalyzes the oxidation of (S)-reticuline to 1,2-dehydroreticulinium ion (44). This enzyme has not been cloned but has been purified partially and shown to be membrane-associated. This intermediate then is reduced by dehydroreticuline reductase, an NADPH-dependent enzyme that stereoselectively transfers a hydride to dehydroreti-culinium ion to yield (R)-reticuline. This enzyme has not been cloned yet but has been purified to homogeneity (45). 

The C = N double bond in the iminium salt of 1,2-dehydroreticuline was enantiospecifically reduced by a (/ )-specific reductase from Papaver somniferum29. 

Now, it is known (Scheme 13.42) that 1,2-dehydroreticuline is reduced by 1,2-dehydroreticulinium reductase (EC 1.5.1.27) with the reduced form of NADPH being oxidized to NADP. However, the enzyme required for the oxidation of (5)-reticuline to 1,2-dehydroreticuline has not yet been characterized. 

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