Alkaloids

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Alkaloids
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Alkaloids

• A large and diverse family of secondary
  metabolites
• Unlike terpenes and phenolics, alkaloids are an
  unrelated group
• About 10,000 alkaloids have been identified in
  about 20 of flowering plants - mostly herbaceous
  dicots

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Other sources of alkaloids

• Relatively rare in gymnosperms and ferns and
  fern-allies
• Not known from mosses and liverworts
• Present in a number of fungi

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Alkaloid distribution in angiosperms

• Some classes of alkaloids are limited to a
  particular family
• A few alkaloids occur in several genera
• However, most alkaloids are restricted to a
  single species
• Often restricted to specific organs within the
  plant

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Alkaloids share some characteristics

• Solubility in water
• At least one nitrogen atom
• Nitrogen often accepts a H which gives slightly
  basic properties, i.e. - alkaloid
• N usually part of a heterocyclic ring

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Role of alkaloids in plants

• A lot of debate - more study is needed
• Many physiologists accept defensive role - they
  have a bitter taste which is considered a
  repellant for all animals
• i.e, nicotine is a potent insect poison
• Many have antibiotic properties suggesting
  defense against microbial infection
• Alkaloid concentrations often highest in most
  vulnerable tissues - young or outermost parts

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Lupine alkaloids

• Bitter taste deters herbivores
• However, grazing on lupines is a common cause of
  cattle poisoning
• Highest concentration of alkaloids occurs in the
  seeds, so livestock losses are highest in the
  fall
• Other alkaloids also poison livestock
• Domesticated animals, especially

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Other alkaloids causing animal poisoning

• Senecionine - groundsel
• Lycotonine - larkspur
• Scopolamine - jimson weed
• Atropine - black henbane

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Other roles in plants

• Some mimic insect pheromones - trick insect into
  visiting the flower for cross pollination
• A few are pigmented (betalains) and act as
  attractants in flowers and fruits

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Effects of alkaloids on humans

• High biological activity
• Produce varying degrees of physiological and
  psychological responses - largely by interfering
  with neurotransmitters
• others interfere with membrane transport, protein
  synthesis or other processes
• In large doses - highly toxic - fatal
• In small doses, many have therapeutic value
• muscle relaxants, tranquilizers, pain killers,
  mind altering drugs, chemotherapy

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Psychoactive alkaloids

• Although only a small percent are psychoactive,
  these get much focus
• Affect the central nervous system - often by
  influencing neurotransmitters
• Categories of psychoactive cmpds
• Stimulants
• Hallucinogens
• Depressants
• May also be narcotic

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Narcotic Compounds

• By definition a narcotic drug induces central
  nervous system depression resulting in numbness,
  lethargy, sleep
• BUT - In current use, a narcotic is any
  psychoactive drug that is dangerously addictive
• Addictive cmpds elicit psychological dependence,
  physiological dependence, and/or tolerance

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Psychoactive alkaloids
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Alkaloid synthesis

• Most alkaloids are synthesized from a few common
  amino acids (tyrosine, tryptophan, ornithine or
  argenine, aspartic acid, lysine)
• Nicotinic acid precursor for part of nicotine
• Purine precursor for caffeine
• Some alkaloids synthesized from terpenes - along
  mevalonic acid pathway
• Generally classified by the predominant ring
  structure and/or carbon skeleton

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Tropane alkaloids
Many of these psychoactive alkaloids
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Pyrrolizidine alkaloids
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Piperidine alkaloids
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Piperidine/Pyridine alkaloids
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Purine alkaloids
Figure 3-5 Caffeine
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Quinoline alkaloids
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Isoquinoline alkaloids
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Indole alkaloids
Vindoline Vinblastine (Catharanthus)
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Quinolizidine alkaloids
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Steroidal alkaloids
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Steroidal alkaloids
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Diterpenoid alkaloids
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Amine alkaloids
Alkaloids without heterocyclic nitrogen atoms
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Other classes of alkaloids

• Betalain alkaloids - pigment alkaloids
• Lycopodium alkaloids - restricted to club mosses
• Peptide alkaloids