Classes of Pharmacologically Active Natural Products

The natural world is full of pharmacologically active chemicals. Most are toxins and anti-feedants produced by plants to discourage predation. Other chemicals are only accidentally biologically active in other species. Although the alkaloids receive the most attention of all the pharmacologically active chemical classes, there are several others as well. Contrary to the selective pressures that led to their production, these chemicals can sometimes be harvested and used as medicines or intoxicants.

Alkaloids

• The large family of bitter-tasting, nitrogenous compounds known as alkaloids are responsible for the activity of the vast majority of natural products and are largely produced by plants as poisons. Their nitrogen group allows many of them to cross the blood-brain barrier. Morphine, one of the most powerful painkillers used in medicine, is an alkaloid. Nicotine is an alkaloid as well, and it is responsible for the addictive properties of tobacco.

Sesquiterpenes

• Sesquiterpenes are a class of terpenes that are produced by plants predominantly as anti-feedants and pheromones. They consist of three isoprene units and have 15 carbon atoms and 24 hydrogen atoms. Sesquiterpenes may be acyclic or contain rings, and they can frequently cross the blood-brain barrier. Consequently, the sesquiterpene parthenolide is one of the few effective drugs in the treatment of migraines, and it is found naturally in the plant feverfew (Tanacetum parthenium). They are also some of the most common drugs used in the treatment of malaria in the form of quinone and hydroquinone.

Alcohols

• Alcohols are carbon-containing compounds in which a hydroxyl functional group binds to a carbon atom that is usually connected to other carbon or hydrogen atoms. Ethanol is a well-known example of a pharmacologically active alcohol through its presence in intoxicating fermented beverages. Closely related methanol is also active; its toxic metabolites can cause blindness and death in those who consume it.

Lactones

• Lactones are an often overlooked class of pharmacologically active natural product. Chemically, lactones can be described as a cyclic ester, effectively the condensation product of an alcohol group and a carboxylic acid group.They possess a closed ring with at least two carbon atoms and a single oxygen atom, with a ketone group in one of the carbons adjacent to the other oxygen.

  Nepetalactone, the active ingredient in catnip (Mentha nepeta), while believed to be inactive or merely mildly sedative in action in human subjects, is powerfully psychoactive in domestic cat species. Kava kava (Piper methysticum) is a culturally significant inebriant used in Pacific Island cultures. Chemicals known as kavalactones are found in large quantities in the root of the plant, producing powerful sedation in those who consume it.