Alkaloids Overview and Functions

Alkaloids

• Alkaloids are natural organic nitrogenous bases
• Primarily found in plants, but also in microorganisms and animals
• Contain at least one nitrogen atom in a heterocyclic ring
• Physiologically active
• Nitrogen atoms are usually secondary, tertiary, or quaternary amines (less commonly primary)
• Classified by the basic nitrogen-containing structures (e.g., piperidine, quinoline, indole)
• Dicots are richer in alkaloids than monocots
• Families rich in alkaloids: Apocynaceae, Rubiaceae, Solanaceae, Papaveraceae
• Families free of alkaloids: Rosaceae, Labiatae

Functions of Alkaloids

• Defense mechanisms, acting as sources of nitrogen or waste
• Mostly have various pharmacological activities
• Central Nervous System (CNS) effects: depressants (morphine), stimulants (caffeine)
• Autonomic Nervous System (ANS) effects: sympathomimetic (ephedrine), sympatholytic (yohimbine), parasympathomimetic (pilocarpine), anticholinergic (atropine), ganglioplegic (nicotine)
• Other activities: anesthesia (cocaine), treatment of fibrilation (quinidine), antitumor (vinblastine), antimalarial (quinine), antibacterial (berberine), amebicide (emetine)
• Starting materials for drug synthesis

Classification of Alkaloids

• Biogenetic classification: based on the biogenetic pathway (e.g., acetate pathway, shikimate pathway)
• Botanical classification: based on the plant source
• Classification by type of amine: primary, secondary, tertiary alkaloids

Basic Alkaloid Skeletons & examples of Alkaloids

• Numerous structures showing various alkaloids
  • Quinoline, Quinazoline, Isoquinoline, Pyrrolidine, Pyridine, Indole, Imidazole, Purine, Isoquinoline, Tropane, Piperidine, Pyrrolizidine, Quinoline, Lupinine

Tropane Alkaloids

• Contain a tropane ring structure.
  • Local anesthetic
  • Mostly used as drugs of abuse generating stimulation and euphoria, but causes depression and dependence.
  • Examples include Hyoscyamine and Cocaine
  • Hyoscyamine has anticholinergic activity, used for antispasmodic and motion sickness prevention.

Piperidine Alkaloids

• Contain a piperidine ring
• Examples include Lobelins; they relieve asthma and bronchitis.

Pyrrolizidine Alkaloids

• Contain a pyrrolizidine ring
• Cause inflammatory, rheumatic, & gastrointestinal (GI) disorders

Quinoline Alkaloids

• Contain a quinoline ring
• Potentially toxic to humans (e.g., sparteine, cytisine, lupinine)

Indoline Alkaloids

• Active against AIDS virus HIV (e.g., Swainsonine, castanospermine)

Pyridine Alkaloids

• Example: Nicotine
• Respiratory stimulant/depressant
• Used in nicotine patches to help with smoking cessation

Tetrahydroisoquinoline Alkaloids

• Example: Anhalamine, Lophocerine
• Examples of the alkaloids involved in the tetrahydroisoquinoline pathway

Benzyltetrahydroisoquinoline Alkaloids

• Example: Papaverine
• Used in expectorants and for male impotence

Modified Benzyltetrahydroisoquinoline Alkaloids

• Examples: morphine, codeine, thebaine (paramorphine)
• Powerful analgesic and narcotic

Indole Alkaloids

• Examples: reserpine, deserpidine, vinblastine, vincristine
• Often found in the plants Rauwolfia serpentina and Vinca minor
• Used in various medicinal applications.

Ergot Alkaloids

• Examples: lysergic acid, ergometrine, ergotamine
• Ergot alkaloids are complex and are used in medicine.

Imidazole Alkaloids

• Example: Pilocarpine
• Used as a miotic eye drop and treatment for glaucoma

Purine Alkaloids

• Examples: Caffeine, Theobromine, Theophylline
• Caffeine is a CNS stimulant and a weak diuretic
• Theobromine is a diuretic and smooth muscle relaxant
• Theophylline is a smooth muscle relaxant for bronchospasm relief