Health benefits of plant alkaloids

Alkaloids are low molecular weight natural organic substances derived from plant sources. Alkaloids are defined as pharmacologically active nitrogen containing basic compounds of plant origin. The word alkaloid means alkali – like or vegetable alkali.  .  The term ‘alkaloid’ was coined by the German pharmacist Carl Friedrich Wilhelm Meissner in 1819 to refer to plant natural products.
 According to R.F. Raffauf (Plant Alkaloids: A Guide To Their Discovery and Distribution, 1996), more than 10,000 different alkaloids have been discovered in species from over 300 plant families.To date over 21,000 alkaloids have been identified.  About 20-30% plant species have been found to contain them. Many of the alkaloids are useful as drugs. The drugs may be either purified natural alkaloids or chemically modified alkaloids.  About 25% of the drugs used today are of plant origin. The plant alkaloids usually have a marked physiological action on man or other animals. The plants use alkaloids as defence chemical agents against herbivores and pathogens. Wink (1993) listed 183 alkaloids with antibacterial properties. The families Solanaceae and Apocynaceae are known for their alkaloid production, 60-70% of their members producing alkaloids. Alkaloid bearing species have been found in nearly all classes of organisms: frogs, ants, butterflies, bacteria, sponges, fungi, spiders, beetles and mammals. Some animals, such as frogs produce toxic alkaloids in the skin or secretary glands.

Ancient  phytotherapy

Alkaloids have been used throughout history in folk medicine in different regions of the world. The alkaloids from plant extracts have been used as ingredients in potions (liquid medicine) and poisons. Ancient people used plant extracts containing alkaloids for treating a large number of ailments including: snakebite, fever and insanity.  Humans have used alkaloids as poisons in weapons. From the times of Hippocrates (460 – 377 BCE), medicinal plants were used in Europe as herbal remedies for improving health. In ancient India and China, herbs were known and used even since 770 BCE. India has a very long, safe and continuous usage of many herbal drugs in the officially recognized alternative systems of medicine viz. Ayurveda, Yoga, Unani, Siddha, Homeopathy and Naturopathy. In India more than 70% of the population use herbal drugs for their health.Millions of Indians use herbal drugs regularly, as spices, home-remedies, health foods, as self-medication or also as drugs prescribed in the non-allopathic systems. The Indian medicinal plants are rich sources of beneficial compounds including antioxidants and components that can be used in functional foods.

Characteristics of alkaloids

The alkaloids are alkaline or basic in chemical reaction. They usually taste bitter. They have complex ring structures (usually with heterocyclic ring) and contain one or more nitrogen atoms. The dietary intake of small doses have  therapeutic effect as muscle relaxants, tranquillizers, pain killers, antimicrobials in human beings. Intake of large doses results in toxaemia or fatal. Many of the alkaloids are synthesized from amino acids such as tyrosine, tryptophan, ornithine, arginine and lysine.   

Physico-chemical properties

Alkaloids (as bases) are not soluble or sparingly soluble in water and soluble in apolar or only slightly polar organic solvents. They are soluble in concentrated hydro-alcoholic solutions. The alkaloids exist in plants in free states or as salts or as N-oxides. Elementally alkaloids consist of carbon, hydrogen and nitrogen. Most alkaloids also contain oxygen.  A few such as coniine from hemlock and nicotine from tobacco are oxygen-free. Most alkaloids have well defined crystalline structures which combine with acids to form salts. Most of the alkaloids have complex molecular structures with significant pharmacologic properties. An alkaloid never occurs alone; alkaloids are usually present as a mixture of a few major and several minor alkaloids of a particular biosynthetic unit, which differ in functional groups (Wink 2005).

Functions of alkaloids in plants

As poisonous agents, alkaloids protect the plants against insects and herbivores.  Alkaloids form as end products of detoxification reactions e.g., secondary metabolites. They serve as regulatory growth factors in plants. They also serve as reserve substances capable of supplying nitrogen or other elements necessary to the plant’s economy.

Biosynthetic origin of alkaloids

Alkaloids are mainly derived from amino acids. The amino acids that most often serve as alkaloidal precursors include phenylalanine, tyrosine, tryptophan, histidine, anthranilic acid, lysine and ornithine. Alkaloids also derived from other precursors like terepenes or steroids.

Naming of alkaloids

The name of all alkaloids ends with the suffix ‘–ine’. In addition the names of the alkaloids are based on various ways.

1.       From the generic name of the plant yielding them e.g., atropine from Atropa bellodona.

2.       From the specific name of the plant yielding them e.g., cocaine.

3.       From the common name of the drug yielding them e.g., ergotamine.

4.       From their physiologic activity e.g., emetine.

5.       From name of the discoverer e.g., pelletierine.

Classification of alkaloids

Typical or true alkaloids have heterocyclic ring with nitrogen derived from amino acids. They can be categorised into 14 groups according to their ring structures e.g., quinine, morphine, codeine etc.,

Atypical or protoalkaloids do not have heterocyclic ring with nitrogen derived from amino acids e.g., colchicines.

Pesudoalkaloids have heterocyclic ring with nitrogen not derived from amino acids. They can be derived from terpenoids or purines.

Chemical classification

Alkaloids are usually classified by their common molecular precursors based on the biological pathway used to construct the molecule.
1. Pyridine group – e.g., piperine, coniine, pilocarpine, nicotine.
2. Tropane group – e.g., atropine, cocaine.
3. Quinoline group – e.g., quinine, strychnine.
4. Isoquinoline group – e.g., opium alkaloids –morphine, codeine.
5. Phenylethylamine group – e.g., ephedrine, mescaline.
6. Indole group – e.g., reserpine, lysergic acid, emetine.
7. Purine group – e.g., caffeine.
8. Terpenoid group – e.g., aconitine.
9. Betaines – e.g., muscarine.

Health benefits

Alkaloids act on the central nervous system either as depressants (morphine) or stimulants (caffeine). They also act on the autonomic nervous system as sympathomimetics ( e.g., ephedrine), sympatholytics (e.g., yohimbine), parasympathomimetics (e.g., pilocarpine), anticholinergics (e.g., atropine) and ganglioplegics (e.g., nicotine). Alkaloids act as local anaesthetics (e.g., cocaine), agents to treat fibrillation (e.g., quinidine), anti-tumour agents (e.g., vinblastine), anti-malarials (e.g., quinine), antibacterials (e.g., berberine), amoebicides (e.g., emetine) and pain killer (e.g., codeine, morphine).