Twenty-five years have passed since Volume 7 ‘‘Secondary Plants Products’’ was
published in the series The Biochemistry of Plants (Waller and Dermer, 1981).
Chapter 12 in that volume, authored by George Waller and Otis Dermer, presented
the current day knowledge of the enzymology of alkaloid metabolism in
plants. Scant information was available on the enzymology of plant alkaloid
formation at the time. Most biosynthetic pathways were inferred from the results
of ‘‘feeding’’ experiments with radiolabeled putative precursor molecules, but did
not involve a study of the enzyme catalysts. Most of the enzymes associated
with alkaloid biosynthesis up until 1979 were decarboxylases, aminotransferases, amine oxidases, and phenol oxidases of relatively broad substrate specificity.
Examples of enzyme activities in cell-free extracts known then that, in retrospect,
were clearly dedicated to alkaloid biosynthesis were the berberine bridge enzyme
(bbe) described by Rink and Böhm in Halle in 1975 (Rink and Böhm, 1975); the
cytochrome P450-dependent geraniol 10-hydroxylase by Coscia and coworkers in
St. Louis in 1976 (Madyastha et al., 1976, 1977); the strictosidine synthase described
by Stöckigt and Zenk in Bochum in 1977 (Stöckigt and Zenk, 1977a,b); and the
strictosidine β-D-glucosidase by Treimer and Zenk in 1978 (Treimer and Zenk,
Immediately after Volume 7 was published in 1981, rapid advances were made
in the ability to detect and purify enzymes involved in plant alkaloid biosynthesis.
This was in large part due to the development of plant cell suspension cultures
and root cultures that produced alkaloids in the tens to hundreds of milligrams
per liter range (Zenk, 1991). Methods of protein purification were greatly
improved using these systems. These protocols could then be applied back to
the native plant systems to identify even more enzymes. In total, in the past
20 years, approximately 80–100 new enzymes of alkaloid biosynthesis have been
identified [summarized in part in Kutchan (1998)].
Central to the advances of the past two decades is also the development of
plant molecular genetic methods. Concurrent to the identification and purification
of new enzymes of alkaloid biosynthesis, the cDNA cloning and functional
expression techniques of plant molecular biology were being developed.
In 1988, the first cDNA of alkaloid formation, encoding strictosidine synthase,
was isolated from Rauwolfia serpentina (Kutchan et al., 1988). One year later, it was
functionally expressed in Escherichia coli (Kutchan, 1989), thus opening a new
period in the study of alkaloid biosynthesis. Other clones have rapidly followed
and today the isolation
and expression of alkaloid biosynthetic genes, although
not yet routine, proceed at an accelerated pace. The full exploitation of these genes
lies in the ability to ultimately engineer alkaloid pathways to generate plants with
tailored alkaloid profiles for basic research and for commercial production. We do
not yet fully understand how alkaloid accumulation is regulated in plants nor do
we have a full complement of plant transformation and regeneration protocols
necessary to producing transgenics of all species that may be interesting or useful.
In retrospect, many aspects of the study of alkaloid metabolism have advanced
tremendously during the past 20 years, but several of the insightful statements of
Waller and Dermer in 1981 remain true today (Waller and Dermer, 1981).
To paraphrase their words, alkaloids play an important role in the ecology and
physiology of the plants in which they occur. Although more easily described
than precisely defined, there are several major incentives for studying alkaloids:
they are the oldest drugs and still have a significant use in modern medicine,
organic chemists are fascinated by the variety and complexity of structures presented,
plant physiologists seek to learn the function of these compounds in
plants, and pharmaceutical industry searches to improve methods of production.
The main difference in the alkaloid field today compared to 1981 is the tools that
are available to aid researchers in analyzing structure, biosynthesis, regulation, function, and production. Our understanding in 2006 of selected medicinally
relevant alkaloid pathways and our ability to manipulate them is as follows.