Plant Secondary Metabolites
Occurrence, Structure and Role in the Human Diet

A. Crozier,
M.N. Clifford,
H. Ashihara, eds.
Blackwell, 2006, 372 pages

econdary metabolites (also called natural products) are molecules made in plants that are not necessary for their survival, but protect plants against insects or infection. A vast number of such compounds are known. They include many of our most important drugs, spices, and perfumes. This book is a collection of mostly excellent review articles on the biochemistry of secondary metabolites and their effects as drugs, nutrients, and toxins. Each of the chapters in this book could easily be expanded into an entire book. The important pathways and structures are illustrated by clearly drawn two-color diagrams.

The three large chapters on alkaloids, terpenes, and acetylenes and psoralens are particularly well-written and informative. In the chapter on terpenes, the authors point out that terpenes are not made from isoprene, as one might suppose, but from isopentenyl pyrophosphate and dimethylallyl pyrophosphate. In the past two decades, it has also become clear that most terpenes in plants are synthesized not through the mevalonate pathway, as was originally believed, but through a pathway starting with 1-deoxyxylulose-5-phosphate---a sugar.

Well-known terpenes include cafestol, found in unfiltered or boiled coffee, which accounts for the ability of boiled coffee to raise the levels of serum cholesterol, and taxol, an anticancer drug that acts by inhibiting mitosis. Another interesting terpene is patchoulol, a constituent of patchouli oil. Patchouli oil was popular with hippies because its powerful scent could mask the smell of body odor and marijuana. Other terpenes have anti-inflammatory, contraceptive, anti-cholesterol, or anti-malarial effects.

The defining characteristic of alkaloids, such as morphine, caffeine, codeine, and nicotine, is the presence of nitrogen. Morphine is unusual because it is synthesized in the vascular system of plants, whereas most others are synthesized in the roots or leaves. The effects of codeine are caused by its demethylation to morphine. Interestingly, many people cannot carry out this biotransformation; for them, codeine has little effect. The alkaloid nicotine is produced only in the roots of tobacco plants. After leaf damage, a chemical signal travels to the roots and induces the synthesis of nicotine, which then travels through the xylem back to the leaves. Another important alkaloid is solanine, found in unripe potatoes. One researcher has theorized that solanine from bad potatoes may be the cause of schizophrenia.

The natural function of another well-known alkaloid, caffeine, is not to keep the coffee plant awake while it is studying for exams, but to protect it against insects (presumably by making them so nervous they fall off the leaf). Caffeine withdrawal, the author says, causes decreased motor behavior. There are many humans who can vouch for the accuracy of that statement.

Other important secondary metabolites are acetylenes (toxic lipids with triple C-C bonds), which are found in carrots, celery and lettuce, and psoralens (furanocoumarins), which are found in celery that has been stressed by fungal infection or pollution. Psoralens bind to DNA and RNA and cause photodermatitis, a light-induced rash, and account for the bitter taste of bad celery.

Other chapters describe sulfur-containing compounds and human intestinal flora, and there is a long, non-technical chapter on secondary metabolites from various individual foods. Most interesting here is one author's theory that intestinal bacteria may contribute to autism. In some of these chapters, the writing could stand to be improved; you will find confused sentences, dangling clauses, and undefined abbreviations, as well as a few factual errors. But for the most part, the book is highly readable. The last chapter is a brief overview of the medicinal chemistry of these molecules.

This book is ideal for biochemists and others who need an overview of natural products and the pathways of their formation. The strength of this book is that most chapters provide not only details on the biosynthesis, but also give a historical and nutritional perspective to the molecules, which broadens its appeal to scientifically literate general readers. However, some background in biochemistry or organic chemistry is recommended to fully benefit from this book.