Protoberberine alkaloids, berberine biosynthesis

Protoberberine Alkaloids.—In the course of the bioconversion of the proto-berberine scoulerine (65) into chelidonine (62) and phthalide-isoquinolines, e.g. narcotine (63), C-13 becomes oxidized.61 Ophiocarpine (68), with a hydroxy-group at C-13, represents an intermediate stage in the modification of the protoberberine skeleton, and results62 of tracer experiments have shown that scoulerine (65) is also to be included in the biosynthesis of this alkaloid. Tetrahydro-protoberberine (67) is also a precursor, its incorporation indicating that C-13 hydroxylation is a terminal step. As for other protoberberine derivatives,63 nandinine (64) was not assimilated,62 and it follows then that (65) is probably converted into (67) by way of isocorypalmine (66). 

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... 

Berberine bridge enzyme catalyses are specialized step in protoberberine alkaloid biosynthesis similar genes and proteins are, however, widely present in higher plants, indicating common ancestry (Fig. 7.17c). BBE or similar proteins, which share a number of common conserved sites (Table 7.8), could also be found in fungi and bacteria. A similar pattern can be seen in the distribution of CR (Fig. 7.17d, Table 7.9). 

Speculations on the biosynthesis of berberine date back to the beginning of the century (121). Most of the early proposals recognized the structural relationship of the protoberberine alkaloids with the simpler benzylisoquinoline bases, from which it was supposed that they are derived. The additional carbon atom necessary for the formal conversion... 

Berberine chloride was evaluated for antimalarial activity against Plasmodium falciparum in vitro (two clones of human malaria Plasmodium falciparum D-6 [Sierra Leone clone] and W-2 (Indochina clone) and Plasmodium berghei in vivo (mice). The alkaloid exhibited an antimalarial potency equivalent to that of quinine in vitro, but was inactive in vivo. The results were consistent with those of others who have found berberine to be a potent inhibitor in vitro of both nucleic acid and protein biosynthesis in P. falciparum, and have demonstrated a strong interaction of berberine with DNA. In addition, the lack of in vivo antimalarial activity in mice observed with berberine and other protoberberine alkaloids agrees with clinical reports that have claimed berberine to be inactive as an antimalarial drug [228]. 

The biosynthesis of protoberberine alkaloids, including berberine, has been extensively studied, and all the enzymes of the biosynthetic pathway have been characterized (11,121,391,508). Interestingly the pathway leading to berberine in Berberis was found to be different from that in Coptis and Thalictrum (509). In the former species berberine is formed from columbamine, in the latter plant species from tetrahydroberberine (121). The production of isoquinoline alkaloids, including the protoberberine alkaloids, by plant cell cultures have been reviewed by Riiffer (390) and Ikuta (510). Table XXVI summarizes patents concerning the production of berberine by means of plant cell cultures. In Table XXVII a summary is given of the occurrence of berberine in some plant cell cultures. 

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). 

Steffens P, Nagakura N, Zenk MH (1984) The berberine bridge forming enzyme in tetrahydro-protoberberine biosynthesis. Tetrahedron Lett 25 951-952 Sumimoto Chemicals Co Ltd (1982) Berberine alkaloid production by tissue culture. Jpn Kokai Tokkyo Koho 57,144,992 (Cl C 12pl7/18)... 

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