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, 1978).
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.
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