Biosynthesis and Metabolism of the Tropane Alkaloids

16 Pages·2008·1.25 MB·English

Journal of
Medicinal
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Plant Research e
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Editor - in - Chief Editorial Board Hippokrates Verlag y pr
PE.h aRreminahzaerudti,s Ucnhievs. Tlnusbtiintugte n WH.. ABmarmz.o nM,u Tnusbteinr gen Stuttgart strictl
A74u0f 0d Teur bMinogregne nstelle E0H... RWS etaiicnghhneaerrr,d, Z,M iTiruuincbhcinh genen Vol. 36 2 ution is
M. Zenk, Bochurn June 1979 NO. strib
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Biosynthesis and Metabolism of the nal
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Tropane Alkaloids per
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Edward Leete . ad
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Natural Products Laboratory, School of Chemistry, University of Minnesota. w
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Key Word Index: Tropane Alkaloids; Biosynthesis; Metabolism;'Tropic'Acid; w
Tiglic Acid; Hydroxylation of Tropine. ent
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Abstract Introduction his
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The origin of both the basic (tropane The tropane alkaloids are characteri-
derivatives) and acidic (tropic acid, tig- zed by the presence in their molecules of
lic acid, benzoic acid) moieties of these the 8-azabicyclo [3.2.1] octane ring sy-
ester alkaloids is reviewed. Subsequent stem (I). Its N-methyl derivative is tri-
metabolism (mostly by oxidation reac- vially known as tropane: Many of the
tions) of the tropine half of these alka- alkaloids are derivatives of tropine (11)
loids is discussed. which is tropan-3a-01. Tropine, having
98 Leete
"j_
I Tropine I1
.:
Hq
, .
d.
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bit
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pr
o\ y
C-CH-Ph O\C-CHCH~-P~ ctl
0II CI H20H 0II OI H s stri
n i
Hyoscyamint Ill Scopolamine IV Littorint V utio
b
stri
di
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e
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ut
,!H ,Me a
n
U
o\c/CH //C nly.
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II eM' e
0 0 s
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Meteloidine V1 Cocaine VII Cochlearine Vlll n
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s
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Bellendine R:H IX 2P -Btnzyl-3P-acttoxy- w
Ferrugine R=Me X 2a-Benzoyltropane XI tropane XI1 nt
e
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' Fig. I. Tropa.ne Alkaloids. ocu
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a plane of symmetry, is optically inac- are illustrated in Fig. 1. Hyoscyamine
tive, however it should be noted that (111) and scopolamine (also known as
C-1 (as depicted in 11) has the R-confi- hyoscine) (IV) are found in a large num-
guration, and C-5 has the S-configura- ber of Datura species. Atropine is the
tion; . and these two positions can be racemic form of hyoscyamine, and race-
different with respect to an enzyme. mization of hyoscyamine usually occurs
Some of the alkaloids whose biosyn- during its isolation from plants. On the
thesis will be discussed in this.review other hand scopolamine seems to be
Biosynthesis and Metabolism of Tropane Alkaloids 99
much more resistant to racemization. mine and scoplamine with little change
Recently discovered alkaloids contain- in the 14C: 15N ratio, while [2-14C,
ing the tropane ring system include bel- a-'5NIornithine yielded tropine con-
lendine (IX) [57], and ferrugine (X) taining much less 15N than 14C. These
[lo] containing a fused y-pyrone ring. results suggest that it is the &amino
Alkaloids (XI) and (XII) are novel in group of ornithine which is a precursor
having a benzoyl or benzyl group at of the tropane nitrogen. However the
C-2 [52, 531. Extensive work involving calculated '4C specific incorporations
d.
labelled precursors has been carried out reported in this publication are too high e
on both the tropane half of these alka- by a factor of 10. If these results are - hibit
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loids, and the various acids (tropic, real they indicate that the d-amino pr
y
tiglic, benzoic) which are present in group of ornithine is ten times more ctl
these alkaloids esterified to tropane al- efficient as a source of the tropane nit- stri
s
cohols. rogen atom, 'than is the carbon ske- n i
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leton of ornithine a source of the uti
b
carbons of the pyrrolidine ring. I stri
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Origin of the Tropane Ring System believe that extensive loss of 15N from d
e
the amino groups'of ornithine occurs by oriz
Ornithine, and related amino acids transamination reactions during the fee- uth
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(glutamic acid, proline) are precursors ding experiments, all of which involved Un
of the pyrrolidine ring.of tropine [26, root cultures. The incorporation of gln- y.
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27, 39, 48, 491 Ornithine is incorpora- tamic acid and proline is considered to e o
ted-into the tropane ring system unsym- occur via ornithine, and the metabolic us
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metrically. It was found that tropine relationship of these amino acids is illu- n
o
(present in hyoscyamine) derived from strated in Fig. 2. SPENSER[5 5] has ers
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[2-14C]ornithine was labelled only at shown that ornithine is converted to or
the C-1 bridgehead carbon. On the proline via a-keto-8-amino valeric acid ed f
d
other hand [5-'4C]proline labels only in Datura species, and presumably th,is oa
the C-5 position of tropine [49]. Work reaction is reversible. wnl
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on the origin of the nitrogen in tropine Figure 3 illustrates how ornithine is s d
a
is equivocal. We [32] found that both converted to tropine. We presently con- w
amino groups OP ornithine were appar- sider that ornithine is incorporated via ent
m
'ently incorporated intohyoscyamineand 8-N-methylornithine (XIV) [I, 2, 31, u
c
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scopolamineproduced in a root culture of although it must be admitted that this d
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Atropa belladonna. However the speci- amino acid has not yet been detected in hi
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fic incorporation of 14C( located at C-2 Datura or related species. Earlier work
'of ornithine) was about three times [58] which .apparently implicated a-N-
higher than the nitrogen (located at the methylornithine rather than the 8-iso-
a- or 8-positions). SPENSER[6 8] found mer as a precursor of tropine, was dis-
similar results. On the other hand LIE- counted when it was shown [2O] that
BI~CHa nd SCHUTTE[4 9] reported that these authors were using non-authentic
[2-14C,d -15Nlornithine was incorpora- a- and S-N-methylornithines in their
ted into the tropane moiety of hyoscya- tracer experiments. It was shown that
100 . Leete
C L
HOOC
Glutarnic acid Glutarnic -y-semialdehyde Ornithinc
m =
0-
HOOC N HOOC N ed.
H bit
hi
Proline A'-Pyrrolinc-2-carboxylic a-~eto-6-arninovalcric pro
acid acid y
ctl
Fig. 2. Metabolic Relationschip Nof GlHutami2c Ac id, Ornichine and Proline. stri
s
COOH - COOH on i
uti
b
cN42 stri
di
NH2 NHMe NHMe NH2 ed
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Ornilhine ori
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Xlll ut
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4-Methylaminbbulaml d f
XIX e
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(+) -(2R)-Hygrinc Dehydrohygrinc Tropinone Tropine Thi
XX XXI XXll 11
Fig. 3. ~etabolisrno f Ornithine to Tropine.
[methyl-14C]-6-N-methyl-[2-14C]~p~orsintiio-n s indicated that the 6-N-methyl-
thine was incorporated into hyoscya- ornithine was incorporated without any
mine labelling C-1 and the N-methyl cleavage of the N-methyl group. The
group. The ratio of activity at these two next step is a decarboxylation to yield
Biosynthesis and Metabolism of Tropane Alkaloids 101
N-methylputrescine (XV) an establi- rine (XX) an established precursor of
shed precursor of tropine [45, 471. Pu- tropine [46, 591. It has been recently
trescine (XVI) has .also been shown to shown [54] that it is the (+)-(2R)-hyg-
be a precursor of the tropane alkaloids rine (XX) enantiomer which serves as a
[23, 36, 48, 501. However putrescine precursor of the tropane alkaloids of
cannot be a free intermediate between Datura innoxia. The incorporation of
ornithine and the tropane skeleton, (-)-(2s)-hygrine, although lower than
'
since such a symmetrical intermediate its 2R isomer (3.7 to 10.7'times less effi-
would result in the formation of tro- cient), was never-the-less significant. d.
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pine labelled equally at C-1 and C-5 The incorporation of the 2s-isomer bit
hi
after the administration of [2-l4C]orni- could be rationalized by suggesting that pro
thine. We suggest that the utilmization of some racemization occurred during -the ctly
putrescine represents an "aberrant" bio- feeding period (7 days). Racemization stri
synthesis. By this we mean that putres- occurs readily in neutral or basic solu- n is
o
cine is not part of the biosynthetic path- tion by the mechanism illustrated in uti
way, however the plant contains enzy- Fig. 4. It is of interest to note that (2R)- strib
mes which are capable.of converting the hygrine is formed by an attack of the di
d
administered putrescine to N-methyl- acetoacetate on the pyrrolinium salt on e
z
putrescine. Oxidation of the primary the opposite side from the one involved hori
ut
amino. group of (XV) affords 4-methyl- in the formation of natural (-)-2's)- a
n
aminobutanal (XIX), which was detec- nicotine (XXIII). This result is illustra- y. U
ted 'in Datura plants which had been ted in Fig. 5. Final steps to yield tropi- nl
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fed [2-'4C]6rnithine [56]. The cyclized none (XXII) are considered to involve se
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form of this amino aldehyde is the N- the hypothet.ica1 intermediate dehydro- al
n
methyl-A'-pyrrolinium salt (XVIII). hygrine (XXI). Tropinone has been iso- so
er
Carbons 2, 3, and 4 of tropine are deri- lated from Cyphomandra species [16]. p
or
ved from acetic acid [24, 461 and it is Stereospecific reduction of tropinone d f
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assumed that the acetate is incorporated yields tropine. d
a
via acetoacetic acid or some suitably It is instructive to follow the biosyn- nlo
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activated derivative such as its coenzy- thetic pathways whereby [l-14C]acetate o
d
me A ester. The result of such a conden- is incorporated into tropine (Figure 6). as
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sation between (XVIII) and acetoace- It has been shown that glutamic acid nt
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tate is hygrine-a-carboxylic acid (XVII) isolated from wheat plants was labelled m
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which on decarboxylation affords hyg- mostly at C-1 and C-5 [Ill. This result oc
d
s
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Fig. 4. Mechanism for:the Racernization of Hygrine.
102 Leete
Nicotinic
acid
\ COOH
d.
e
bit
hi
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pr
y
ctl
\M e n is stri
Me utio
b
stri
di
d
Fig. J. Nucleophilic Attack of N-Methyl-dl-pyrrolinium Salt from the a- and p-face. e
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CH~EOOH CH~~OOH - ~ H ~ ~ O O H onl
PCOOH I I e
O=C-IC OOH HO-C-CI OOH +++ CIH,2 - ---- CI H~~OOH O=C-ICO.OH al us
CH2COOH CH2COOH 04C O:tOH CH2COOH CHZCOOH on
s
er
Oxaloacctalc Citratc a-K~togIuIarate Succinale or p
d f
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d
a
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- - wnl
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,COOH CI HzCOOH CIH2 COOH CIH.~~ OOH s d
C/cH;-c2H-\ CH2 .NH2 C1H2 .. C1 H2 . HO-C-CIOO H nt wa
'NH~ ,CH-COOH ,C-COOH CH~EOOH me
H2N 0 ' u
c
o
d
s
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Tropine
Fig. 6. The Incorporation of [1-14C]Acetate into Tropine.
Biosynthesis and Metabolism of Tropane Alkaloids 10 3
is in accord with the expected incor- [46] yielded radioactive cocaine. Ho-
poration of acetate into the Krebs cycle. wever most of the activity (- 60b/o)
The ornithine derived from this gluta- was apparently located on the ester me-
mate would be labelled at C-1 and C-5. thyl 'group. Another 30 O/o was present
Incorporation of this ornithine as pre- on the carbonyl group of the benzoyl
viously described yields tropine label- residue, and only small amounts were
led at the C-5 bridgehead position with present in the tropane ring system. We
no label at C-1 [12]. Labelling at C-3 [42] and others [69] have also failed to
d.
results from the incorporation of the observe the incorporation of likely e
bit
[I ,3-i4C]acetoacetate, and the higher precursors (putrescine, ornithine, N- hi
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level of activity found at this position methyl-dl-pyrrolinium salt) into the pr
y
(81 010) is in accord with the administer- pyrrolidine ring of cocaine. ctl
ed acetate being closer biochemically to Reports [25] that labelled succinic s stri
this part of the molecule. acid was incorporated into hyoscyami- n i
o
It seems reasonable to propose that ne, led BARELLaEn d GROS[ 3] to repeat uti
b
the tropane moiety of cocaine (VII), this experiment, with the finding that stri
di
ie. ecgonine (XXIV) is produced by a non-specific labelling occurred. d
e
slight modification of the biosynthetic The tropane alkaloids having a C,- oriz
route illustrated in F.ig. 3, whereby the C, group attached to C-2 of the tropane uth
a
carboxyl group of hygrine-a-carboxylic molecule are plausibly derived by reac- n
U
acid (XVII) is retained, as illustrated in tion of the N-methyl-dl-pyrrolinium y.
nl
Fig. 7. However no definitive results salt with a poly-ketide having benzoic o
e
have been obtained on.t he biosynthais acid as its starter unit, as illustrated us
al
of the tropane moiety of this important schematically in Fig. 8. This Figure also n
o
medicinal agent. The administration of depicts the formation of bellendine (IX) ers
p
[l-14C]acetate to Erythroxylon coca from the pyrrolinium salt and a poly- or
d f
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d
a
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COOH COOH o
d
s
a
w
nt
e
m
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c
o
d
s
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Cocaine Ecgonine
VII XXIV
Fig. 7. Hypothetical Biosynthesis of Cocaine.
104 Leete
Ph Ph
COOH
qcn2;
NHMe
d.
e
bit
4 : - P 0h 0-C-Me hi
o
pr
H y
ctl
stri
s
n i
o
uti
b
stri
di
d
e
z
ori
h
ut
Bellendine IX a
n
U
Fig. 8. Hypothetical Biosynthesis of Some Novel Tropane Alkaloids. y.
nl
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s
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acetyl chain which has undergone C- 3a-tigloyloxytropan-7B-01 (XXVII). nal
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methylation (from methionine) at a rea- One problem in studying the origin of ers
p
sonable position (between two carbonyl these tigloyloxytropanes is the facile or
groups). hydrolysis which these compounds un- d f
e
d
dergo in the plant in the course of fee- a
o
ding experiments. The dihydroxylation nl
w
o
Oxidation of the Tropane skeleton which occurs at C-6 and C-7 in the for- d
s
mation of meteloidine proceeds with wa
It was shown that the pyrrolidine retention of configuration [38]. This re- nt
e
m
ring of meteloidine (VI) is derived from sult is typical of direct hydroxylations u
c
ornithine [41] and the hydroxyl groups at saturated carbon atoms [7], and was do
s
at the 6 and 7-positions are apparently established by. labelling tropine stereo- hi
T
introduced at a late stage in the biosyn- specifically wtih tritium at the 68 and
thetic sequence, after formation of the 7B-positions.
tropine ring system [28]. Extensive A related oxidation of the tropine
work by WOOLLEYan d coworkers [6, ring system occurs in the formation of
8, 91 and ourselves [37] indicate that scopolamine. Long ago it was esta,bli-
meteloidine is formed by the hydroxy- shed that this alkaloid is a metabolite
lation of 3a-tigloyloxytropane (XXV) of hyoscyamine [19, 63, 64, 65, 661,
(also known as tigloidine), probably via and the reaction proceeds via 6-hydro-
Biosynthesis and Metabolism of Tropane Alkaloids 105
Tmpine I1 3a-Tigloyloxytropane XXV
d.
e
bit
hi
o
pr
y
ctl
H stri
0 Tig 0 Tig n is
o
uti
b
stri
di
d
e
z
ori
h
ut
a
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U
y.
nl
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s
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XXVlll Meteloidine V-1 al
n
o
Fig. 9. Biosynthesis of Some Tigloyloxytropanes. ers
p
or
d f
e
d
a
o
- nl
w
o
0 TroHp yl 0 TroHp yl as d
w
nt
Hyoscyamine 111 6-Hydroxyhyoxyamine XXX XXXI e
m
u
c
o
d
s
6q hi
T
qH
G
H Enyme-S
0 Tropyl 0 Tropyl 0 Tropyl
Scopolamine 1V . 6,7-Dehydrohyoscyomine XXXll XXXlll
Fig. 10. Metabolism of Hyoscyamine to Scopolamine. .
Leete
xyhyoscyamine (XXX) and 6,7-dehy- scopolamine revealed the presence of
drohyoscyamine (XXXII). This overall satellite peaks, due to spin-spin coupling
reaction also involved the loss .of both of contiguous 13C atoms which arose by
the ,B-hydrogens at C-6 and C-7 [38]. a 1,2-intramolecular rearrangement.
This result indicates that the conver- The mechanism of this rearrangement
sion of 6-hydroxyhyoscyamine to the is unknown. Compounds which are me-
dehydro-compound (XXXII) involves tabolically related to phenylalanine,
a cis-dehydration. This is unusual, but such as phenylpyruvic acid (XXXV)
not without precedence [13]. We sug- and phenyllactic acid (XXXIV) are ed.
gest that the dehydration takes place by also incorporated into tropic acid [17, hibit
o
a two step mechanism illustrated in Fig. 331. Phenyllactic acid is esterified with pr
y
10. A phosphorylated 6-hydroxyhyos- tropine in the alkaloid littorine (V) and ctl
cyamine (XXXI) could be attacked by its formation from phenylalanine has stri
s
a nucleophilic reagent (represented here been established [I 83. When phenylala- n i
o
by the SH-group of an enzyme) to af- nine was labelled with tritium in its side uti
b
ford (XXXIII). Then the double bond chain it was found [67] that the tritium stri
would be generated by a trans-elimina- at C-2 was not incorporated into tropic d di
e
tion. acid, however one of the tritiums at z
ori
C-3 was retained in the ultimate tropic h
acid [43]. DALTO[N15 ] has proposed a aut
Origin of the Acid Moieties of the Un
Alkaloids rgeeamsoennat,b alen dh yitp oist hreepsirso dfoucr etdh iisn rFeiagr.r a1n1-. only.
e
Tropic Acid and Related Aromatic One experimental fact which this sche- s
u
Acids me does not take into account is the fin- al
n
o
Tropic acid having the (S)-configura- ding [31] that 2-formylphenylacetic - ers
tion (XXXRIII) is found in the alka- acid (XXXVII) failed to serve as a pre- or p
loids hyoscyamine and scopolamine. Its cursor of tropic acid in Datura. d f
e
biosynthesis has been the subject of ex- The same rearrangement of the phe- d
a
o
tensive investigations by several groups nylalanine side chain is involved in the nl
w
in the past 18 years. This work has biosynthesis of the fungal product ten- do
been reviewed [29] and I will confine ellin (XL). The intramolecular nature as
w
my remarks to recent developments. of this reaction was also established by nt
e
Tropic acid is formed by the rearrange- means of I3C NMR [35]. 2-Formylphe- um
c
ment of the side chain of phenylalanine nylacetic acid, or its coenzyme A ester o
d
(XXXVI) and it has been shown that (IXL) would be a plausible interme- his
T
this rearrangement is intramolecular diate to condense with a polyketide
[MI. This important result was obtain- chain derived from acetate and the me-
ed by labelling phenylalanine at C-1 thyl groups of methionine as illustrated
and C-3 with carbon-13, so that the ad- in Fig. 12. Hydroxylation of the phenyl
ministered amino acid contained 81°/o ring apparently occurs at a late stage in
of the doubly labelled species. Exami- the biosynthesis.
nation of the ISC NMR spectra of the Despite claims to the contrary [22,
resultant labelled hyoscyamine and 701 we [34] have been unable to sub-