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Last updated date: 24th Jul 2024
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What is Alkaloid?

You can find many natural compounds in nature. Alkaloids is a special class among many classes of naturally occurring organic compounds such as carbohydrates, proteins, lipids, amino acids, anthocyanins, flavonoids and steroids. The thing which makes them special is that they are derived from amino acids. These can be synthesised by plants and some of the animals as secondary metabolites. These compounds are very important for living organisms. Alkaloids have been extremely important for human beings for ages, besides they are secondary metabolites, which suggests that they are useless.   Alkaloid shows strong biological effects on humans and animals also in very small amounts. Alkaloids are present in human life also. They are also used as stimulant drugs.

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Alkaloids meaning can be defined as any naturally occurring organic nitrogen-containing class bases. Alkaloids contain diverse and essential physiological effects on humans and on other animals. A few of the well-known alkaloids can be given as strychnine, morphine, ephedrine, quinine, and nicotine.

Alkaloids originated from the word alkali. They react with acids and form the salts like the inorganic alkalis. In an acid-base reaction, these nitrogen atoms behave as a base. In general, alkaloids are also treated as amines. Although, it has the suffix -ine. Alkaloids in pure form are usually colourless, odourless crystalline solids, but sometimes they can be yellowish liquids. Quite often, they also have a bitter taste. 3000 alkaloids are known in more than 4000 plant species.

Definition of Alkaloid

Pelletier defined an alkaloid as: “a cyclic organic compound containing nitrogen in a negative oxidation state which is of limited distribution among living organisms”.

Alkaloid: An alkaloid is a group of chemicals that is made from plants. It contains nitrogen in them. Many alkaloids also have potent pharmalogic effects. Some of the examples of alkaloids are cocaine, nicotine, strychnine, caffeine, morphine, pilocarpine, atropine, methamphetamine, mescaline, ephedrine, and tryptamine.

Nature of Alkaloids: Basic Aspects

Some alkaloids are also poisonous. The poisonous activities of many alkaloids are known for ages. One of its examples is strychnine. It is derived from Strychnos species. It is one of the well-known poison curare. It is obtained from the Chondrodendron tomentosum species. It is used in South Africa as a narrow poison. It contains alkaloid tubocurarine.

Coniine is also an alkaloid that is isolated from Conium maculatum. Coniine is an active ingredient of hemlock poison. 

Mescaline isolated from Anhalonium species has hallucinogenic activity. Psilocybin is also a naturally occurring drug. It is isolated from fungi species Psilocybe Mexicana and possesses psychedelic activity. Also in previous decades, many semisynthetic derivatives of naturally occurring alkaloids with various activities have been synthesised. The heroine is one of the synthetic derivatives of morphine. It is prepared naturally from Lysergic acid present in C. 

Availability of Alkaloids

Alkaloids can be found majorly in plants and are especially in certain families of flowering plants. These are called plant alkaloids. In fact, as several as one-quarter of higher plants are estimated to have alkaloids, of which many thousand various types of alkaloids have been identified. Generally, a given species have only a few kinds of alkaloids, though both the ergot fungus (Claviceps) and opium poppy (which is Papaver somniferum) each contain up to 30 various types.

Certain plant families are specifically rich in alkaloids, where, for example, all plants of the poppy family (which is Papaveraceae) are thought to contain them. The Solanaceae (otherwise called nightshades), Amaryllidaceae (otherwise called amaryllis), and Ranunculaceae (otherwise called buttercups) are other prominent alkaloid-containing families. Some alkaloids have been found in the species of animals, such as the poison-dart frogs (otherwise called Phyllobates) and New World beaver (otherwise called Castor canadensis). Ergot and some other fungi also produce them.

Alkaloids - Their Importance in Nature and for Human Life

Alkaloids function in plants is yet to be understood. It has also been suggested that they are simply waste products of the metabolic processes of plants, but the evidence suggests that they can serve particular biological functions. In a few of the plants, alkaloids' concentration gets increased just before the seed formation and after that, it drops off when the seed gets ripened, suggesting the way that alkaloids can play a role in this process. Alkaloids can also protect a few plants from the destruction by certain insect species.

Characteristics of Alkaloids

Almost several alkaloids have oxygen in their molecular structure, where such compounds are said to be colourless crystals at ambient conditions, in general. Oxygen-free alkaloids, like coniine or nicotine, are typically colourless, volatile, and oily liquids. A few alkaloids are coloured, like sanguinarine (orange) and berberine (yellow).

When swallowed, most alkaloids have a sour flavour or are toxic. Plants' production of alkaloids appeared to have evolved in response to herbivorous animal feeding. However, a few animals have evolved the ability to detoxify alkaloids. Some can result in the production of developmental defects in the offspring of the animals that result in consuming but cannot detoxify alkaloids. An example is given as alkaloid cyclopamine, which is produced in corn lily leaves. 

In the 1950s, around 25% of lambs, born by the sheep that contained grazed corn lilies, had several serious facial deformations. These ranged from the deformed jaws to cyclopia. The responsible compounds for these deformities were later known as alkaloid 11-deoxy jervine, which was later called cyclopamine, following decades of study in the 1980s.

Classification of Alkaloids

Classification based on the botanical origin of the alkaloids are also used, e.g., Papaver (opium) alkaloids, Cinchona alkaloids, Rauvolfia alkaloids, Catharanthus alkaloids, Strychnos alkaloids, Ergot alkaloids, cactus alkaloids, and Solanum alkaloids.

Often, alkaloids are classified based on their chemical structure. 

For example, alkaloids that contain a ring system, which is called indole, are referred to as indole alkaloids. Based on the principal classes of alkaloids are the tropanes, pyrrolidines, indoles, isoquinolines, and steroids and terpenoids. In other ways, alkaloids are classified based on the biological system where they take place. For example, the opium alkaloids take place in the opium poppy (which is Papaver somniferum). Actually, this dual classification system produces a little confusion because there's a rough correlation between the biological distribution of alkaloids and their chemical types.

Often, the alkaloids are divided into the following primary groups:

  • The alkaloids which are called "True alkaloids" contain the nitrogen content in the heterocycle and it originates from the amino acids. The characteristic examples of alkaloids are nicotine, morphine, and atropine. Also, this group includes a few alkaloids that, besides nitrogen heterocycle, contain peptide fragments (for example, ergotamine) or terpene (for example, evonine). The piperidine alkaloids coniceine and coniine can be regarded as the true alkaloids, although they do not originate from the amino acids.

  • Polyamine Alkaloids- The derivatives of spermidine, spermine, and putrescine.

  • Cyclopeptide and peptide alkaloids.

  • Pseudoalkaloids- The alkaloid-like compounds that don't originate from the amino acids. These groups are steroid-like and terpene-like alkaloids and purine-like alkaloids as well such as theobromine, theophylline, and caffeine. A few of the authors have classified the pseudo alkaloids compounds such as cathinone and ephedrine. Those particular alkaloids originate from the phenylalanine amino acid. But, they acquire their nitrogen atom not from the amino acid but via transamination.

A few alkaloids don't contain the carbon skeleton characteristic of their group. Therefore, homoaporphines and galanthamine do not contain isoquinoline fragments, but, in general, they are attributed to the isoquinoline alkaloids.

Extraction of Alkaloids

Since alkaloids have such a wide structural diversity, there is no way to remove them from natural raw materials. And, most of the methods which are available exploit most of the alkaloid's properties to be soluble in the organic solvents, but not in the water, and the opposite tendency of their salts.

Most plants have many alkaloids. First, their mixture can be extracted, and then, the individual alkaloids are separated. Plants are thoroughly ground prior to extraction. Most of the alkaloids, which are present in the raw plants in the salt form of the organic acids. The extracted alkaloids can also remain salts or change into bases.

Pharmaceutical and Medicinal use of Alkaloids

Alkaloids showed quite diverse medicinal properties. Most of these properties are local anaesthetic properties. Although, their chemical use is just limited to clinical purposes. One of the most known Alkaloids is morphine which is used for medical purposes. Morphine is an important Alkaloid that is also used to relieve pain. It has also a disadvantage as it is an addictive drug. 

Methyl ether is the derivative of morphine codeine naturally. It is next to morphine in opium poppy. It possesses an excellent analgesic activity and is known as relatively non-addictive. These alkaloids also act as respiratory and cardiac stimulants. Also, the alkaloid used for medical purposes and has various applications in clinical trials is atropine. The injection with atropine is given to treat bradycardia.

Tubocurarine is an alkaloid that is the ingredient of poison curare. It is also used in surgery as a muscle relaxant. Chemotherapeutic agents Alkaloids vincristine and vinblastine are used in the treatment of many cancer types. An alkaloid present in Erythroxylum coca, which is a potent local anaesthetic is Cocaine. 

Ergonovine is an alkaloid made from fungus Claviceps Purpurea. The Ephedra derived ephedrine is used as a vessel constrictor along with the Ergonovine.  Ephedrine is also used in bronchial asthma. It is also used to relieve the discomfort of hay fever and sinusitis.

Quinine is also a powerful antimalarial agent. However, it can be replaced by synthetic drugs. It is very effective and less toxic. Quinidine is another alkaloid of Cinchona species.  It has numerous medical applications in the treatment of irregular rhythms of the heartbeat which is also known as arrhythmias. 

Colchicine is also an alkaloid of the Liliaceae family. It is known for the ages to treat acute gout attacks. Lobeline is another alkaloid. It is isolated from the Lobelia inflata. Which has multiple action mechanisms.

Use of Alkaloids in Food and Drinks

Many alkaloids are elements of the human diet, food and drinks. Some of its examples in which alkaloid is present are coffee seed, cacao seed and tea leaves. Also, tomatoes (tomatine) and potatoes (solanine) contain alkaloids. Caffeine is the most common alkaloid. It has also an application as an ingredient of soft drinks like Coca-Cola to improve their taste.

Also, it is used in sports drinks to keep the sportsmen energetic.

Practical use of Alkaloids

Other than these properties, alkaloids from many different plant species have many other useful applications. These are such as : 

  • Antiparasitic, 

  • Antiplasmodial, 

  • Anticorrosive, 

  • Antioxidative,

  • Antibacterial, 

  • Anti-HIV, 

  • Insecticidal activities.

FAQs on Alkaloid

1. Explain the biosynthesis of Alkaloids?

Biological precursors of several alkaloids are amino acids, like lysine, phenylalanine, anthranilic acid, and aspartic acid. Nicotinic acid is synthesized from aspartic or tryptophan acid. Alkaloid biosynthesis ways are more numerous and cannot be classified easily. However, there are some typical reactions involved in various classes of alkaloid biosynthesis, including the synthesis of the Mannich reaction and Schiff bases.

2. Give the medicinal applications of Alkaloids.

The use of alkaloid-containing plants in medicine has a long tradition, but when the first alkaloids were discovered in the 19th century, they were quickly put to use in clinical practice. Still, several alkaloids are used in medicine, generally in the salt form.

3. Give the agricultural uses of Alkaloids.

Before the development of extensive, relatively low-toxic synthetic pesticides, a few alkaloids, like salts of anabasine and nicotine, were used as insecticides. Their uses were limited by the human's high toxicity.

4. Explain the use of Alkaloids as psychoactive drugs.

Plant preparations containing later pure alkaloids and alkaloids, as well as their extracts, have been used as psychoactive agents for a long time. Caffeine, cathinone, and cocaine are stimulants of the central nervous system. Mescaline and several indole alkaloids have hallucinogenic effects. Codeine and morphine are strong narcotic pain killers.

5. What are the benefits of Alkaloids?

Alkaloids are important chemical compounds that serve as a rich reservoir for drug discovery. Several alkaloids isolated from natural herbs exhibit antiproliferation, antibacterial, antiviral, insecticidal, and antimetastatic effects on various types of cancers both in vitro and in vivo.