The biosynthesis of papaverine proceeds via (S)-reticuline

Abstract

Papaverine is one of the earliest opium alkaloids for which a biosynthetic hypothesis was developed on theoretical grounds. Norlaudanosoline (=tetrahydropapaveroline) was claimed as the immediate precursor alkaloid for a multitude of nitrogen containing plant metabolites. This tetrahydroxylated compound was proposed to be fully O-methylated. The resulting tetrahydropapaverine should then aromatize to papaverine. In view of new experimental data, this pathway has to be revised. Feeding of 8-day-old seedlings of Papaver followed by direct examination of the metabolic fate of stable isotope-labeled precursors in the total plant extract, without further purification of the metabolites, led to elucidation of the new papaverine pathway in vivo. The central and earliest benzylisoquinoline alkaloid is not the tetraoxygenated norlaudanosoline but the trihydroxylated norcoclaurine that is further converted into (S)-reticuline, the established precursor for poppy alkaloids. The papaverine pathway is opened by the methylation of (S)-reticuline to generate (S)-laudanine. A second methylation at the 3′ position of laudanine leads to laudanosine, both are known alkaloids from the opium poppy. Subsequent N-demethylation of laudanosine yields the known precursor of papaverine: tetrahydropapaverine. Inspection of the subsequent aromatization reaction revealed a new intermediate, 1,2-dihydropapaverine, which has been characterized. The final step to papaverine is the dehydrogenation of the 1,2-bond, yielding the target compound papaverine. We conclusively show herein that the previously claimed norreticuline does not play a role in the biosynthesis of papaverine.

Characterization of a Flavoprotein Oxidase from Opium Poppy Catalyzing the Final Steps in Sanguinarine and Papaverine Biosynthesis

“…The imine-to-enamine isomerization is followed by the introduction of a double bond at C1ϭN2 to form papaver-ine. Radioactive tracer experiments with opium poppy seedlings revealed the presence of 1,2-dihydropapaverine upon feeding (RS)-tetrahydropapaverine or (RS)-laudanosine in support of this mechanism (41). Whether DBOX-generated dihydropapaverine represents the 1,2-dihydro or 3,4-dihydro isomer is not known because the purported diagnostic fragment ions following CID analysis (m/z 190 and 204, respectively) (41) were detected at equally low abundance (supplemental Fig.…”

“…3). CID analysis of dihydropapaverine revealed low abundance, approximately equal intensity ions of m/z 190 and 204, which represent signature fragment masses of 1,2-dihydropapaverine and 3,4-dihydropapaverine, respectively (41). DBOX exhibited the highest relative activity with dihydrosanguinarine, producing the fully conjugated product sanguinarine (Figs.…”

Mu Opioids and Their Receptors: Evolution of a Concept

“…The question arose whether the endogenous morphine is synthesized in the brain or whether it is absorbed from foodstuffs, particularly with its isolation from milk and plants other than the poppy (Hazum et al, 1981b). Although ingestion remains one potential mechanism for accumulation of endogenous morphine, there is evidence it can be synthesized de novo by mammals (Goldstein et al, 1985;Weitz et al, 1987;Poeaknapo et al, 2004;Boettcher et al, 2005;Grobe et al, 2010) and even in isolated cells (Poeaknapo et al, 2004;Boettcher et al, 2005;Han et al, 2010). In addition to morphine, other metabolites have been isolated from brain, including morphine-6b-glucuronide and morphine-6-sulfate (Fig.…”

Benzylisoquinoline alkaloid biosynthesis in opium poppy

“…The efficient incorporation of radiolabeled N-desmethyl compounds (Brochmann-Hanssen et al 1975;Uprety et al 1975) and the identification of a norreticuline 7-O-methyltransferase (N7OMT), which accepts only norreticuline yielding norlaudanine (Pienkny et al 2009), provides biochemical support for the N-desmethyl pathway. A recent study using mass spectrometry and heavy stable-isotope labeling of (S)-reticuline, (S)-laudanine and (S)-laudanosine suggested that N-methylated 1-benzylisoquinolines are incorporated into papaverine at low levels (Han et al 2010), providing some support for the N-methyl pathway. However, most of the labeled (S)-reticuline was incorporated into morphinan alkaloids.…”