Introduction to Alkaloids
Introduction to alkaloids
At least 10,000 alkaloids are known; about 16,000 are listed in the NAPRALERT database. They are found in many plant families. Perhaps 20-30% of all plant species accumulate alkaloids. They are much more common in families such as the Solanaceae and Apocynaceae where as many as 60-70% of species accumulate alkaloids.
Many alkaloids are used in medicine. They sometimes are very expensive (e.g., vincristine, which sells for as much as $22,000 per gram.
Many alkaloids are toxic. They often are responsible for the properties of plants poisonous to livestock.
What is an alkaloid?
Most alkaloids are derived from a few common amino acids. Mmost have physiological activity. Most are basic, but have non-basic forms, such as quaternary compounds and N-oxides.
Many alkaloids give simple color and precipitation reactions, such as with Dragendorf's reagent, that make it relatively easy to determine their presence or absence in plant material.
As a group alkaloids are easily isolated. Their basic and lipid properties make it possible to extract them in acidic forms and to separate them from other water soluble materials. Usually they are extracted with lipophilic or non-polar solvents such as chloroform or petroleum ether, the solutions partitioned with dilute aqueous acid, washed with a non-polar solvent, basicified and again extracted with a non-polar solvent to take up the mixture of alkaloids. Further separation and purification of the resulting mixture of alkaloids is often much more complex, however.
Biosynthetic origins of alkaloids
In a similar way to other plant compounds, many alkaloids appear to undergo "turnover" or synthesis and catabolism, whereas others do not. Diurnal variation in alkaloid content has been demonstrated in a number of plants.
Most alkaloids in plants arise from ornithine, lysine, nicotinic acid, anthranilic acid,
phenylalanine, tyrosine, and tryptophan.
General steps in biosynthesis
Biological activity
Because of the similarity of many alkaloids and neurotransmitters found in many animals, alkaloids often interact with the nervous system. Among these neurotransmitters are acetylcholine, noradrenaline, dopamine, serotonin, and gaba. These involve both CNS-ANS sites.
However, other alkaloids have saponic activity, some disrupt membranes, some are neurotoxic, a number
interact with other types of receptors. In summary, alkaloids have many diverse kinds of biological
activity.
Common Rections in Alkaloid Biosynthesis
Lecture Slides
© David S. Seigler, Plant Biology 363, Plant Secondary Metabolism, Department of Plant Biology,
265 Morrill Hall, 505 S. Goodwin Ave., University of Illinois, Urbana, Illinois 61801, USA.
217-333-7577. seigler@life.uiuc.edu.
