Amines and alkaloids

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Chapter 23 Amines and alkaloids

Chapter contents

Amines 175
Alkaloids 176

This group of pharmacologically active substances is possibly the best known in terms of the effects its members have had on the human population: from Socrates’ death by hemlock to the socioeconomic problems caused by heroin and cocaine. This group is now, however, being investigated for cancer treatment.

The distinction between alkaloids and amines is not very clear in some cases, but a general definition of an alkaloid is said to be an amine isolated from a plant.

Amines

• Putrescine (Figure 23.1) is a well-known amine found in decaying material.
• The neurotransmitter, serotonin, is a tryptamine (see Figure 31.6, p. 241).
• Histamine is involved in inflammation and the immune system (see Chapter 30 ‘Inflammation and the immune system’, p. 225 and Chapter 34 ‘Respiratory diseases’, p. 267).
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Figure 23.1 Chemical structures of three well-known amines.

Alkaloids

Thousands of different alkaloids have been discovered. They are most commonly found in the following groups:

• Apocynaceae: Vinca spp. (periwinkle).
• Berberidaceae: Berberis spp. (barberry), Epimedium spp. (Yin Yang Huo), Coptis spp. (Huang Lian).
• Fabaceae: Vicia faba (fava bean).
• Papaveraceae: Papaver somniferum (opium poppy), Chelidonium majus (greater celandine), Cordydalis spp. (Yan Hu Suo).
• Ranunculaceae: Aconitum spp. (aconite, Fu Zi), Pulsatilla spp. (pasqueflower, Bai Tou Weng), Chelidonium majus (celandine).
• Rubiaceae: Cephaelis spp. (ipecacuanha), Cinchona spp. (cinchona), Coffea spp. (coffee).
• Solanaceae: Atropa belladonna (deadly nightshade), Capsicum spp. (bell peppers and chilli peppers), Datura stramonium (thorn-apple).

Fungi are also well known for their production of alkaloids. The ergot fungus was responsible for the poisoning of thousands of people during the Middle Ages because of its inclusion in rye bread as a result of infected grain. The psychogenic properties of various mushrooms are due to alkaloids.

There are three types of alkaloid (Figure 23.2):

1. True alkaloids:

• Are always basic in nature (see Chapter 8 ‘Acids and bases’, p. 54).
• Contain one or more nitrogen atoms in a heterocyclic ring (the ring contains molecules of different elements) (see Chapter 2 ‘The atom’, p. 7).
• Are synthesized from an amino acid.

2. Protoalkaloids:

• Nitrogen atom is not heterocyclic, e.g. colchicine, ephedrine, muscarine.
• Are synthesized from an amino acid.

3. Pseudoalkaloids: are not biosynthesized from amino acids, e.g. caffeine and theophylline originate from a purine base (see Figure 12.1, p. 90).
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Figure 23.2 The different structures of the three different types of alkaloid.

Alkaloids are classified according to:

• The amino acid that provides both the nitrogen atom and the basic structure of the alkaloid skeleton.
• Chemical origins of alkaloids (Figures 23.3 and 23.4):

– ornithine: trophane, pyrrolidine, pyrrolizidine
– tryptophan: indole, quinoline
– tyrosine: benzyl-iso-quinoline, isoquinoline
– lysine: quinolizidine, piperidine
– pyridine: pyridine.
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Figure 23.3 The chemical origins of various alkaloids.

image image image image

Figure 23.4 T he derivation of various groups of alkaloid.

There are also variations:

• Amine alkaloids: alkaloids without heterocyclic nitrogen atoms, e.g. muscarine.
• Terpenes: diterpenoids (aconitine), steroids (solanine).
• Betalain alkaloids: pigment alkaloids.

Characteristics of Alkaloids

Alkaloids are chemically very active and can produce large effects at small doses, either providing a therapeutic or fatal response. They interfere with:

• Neurotransmitters: see Chapter 31 ‘The nervous system’ (p. 240).
• Active membrane transport: see Chapter 16 ‘How do drugs get into cells?’ (p. 125).
• Protein synthesis: see Figure 11.6 (p. 85).

Therapeutic Use

• Tranquillizers: Zizyphus spinosa (sanjoinine, Suan Zao Ren), Papaver spp. (opium poppy).
• Pain killers: Papaver spp. (opium poppy).
• Muscle relaxants: Chondodendron tomentosum (tubocurarine).
• Anticancer: vinblastin.

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