3. Types and Mechanisms of Hormonal Action

3.1. Types of Hormones

  1. Peptide and Protein Hormones
    • consist of multiple amino acids that link to form an amino acid chain.
    • Peptide hormones consist of short chains of amino acids, whereas protein hormones are longer polypeptides.
    • Both types are synthesized like other body proteins: DNA is transcribed into mRNA, which is translated into an amino acid chain.
    • Act on cell surface receptors
    • Examples of peptide hormones: insulin, glucagon, ADH
    • Examples of protein hormones: GH, FSH
  2. Steroid hormones
    • are derived from the lipid cholesterol.
    • Structure: cholesterol derivatives, lipids
    • Solubility: lipophilic
    • Synthesis: stepwise modification of cholesterol molecule in various intracellular compartments (in the endocrine gland and also in the blood and target tissues)
    • Storage: hormones not stored, only precursor (cholesterol) stored
    • Secretion: diffusion
    • Transport: mostly bound to plasma proteins
    • Receptor site:  inside of target cell
    • Action: direct effects on genes ® production of new proteins
    • e.g., cortisol, estrogen, aldosterone.
  3. Amino Acid Derivatives (Amine hormones)
    • are synthesized from the amino acids tryptophan or tyrosine
    • Catecholamines (dopamine, epinephrine, and norepinephrine)
  • Structure: tyrosine derivative
  • Solubility:hydrophilic
  • Synthesis: cytosol
  • Storage: secretory (chromaffin) granules
  • Secretion: exocytosis
  • ransport: as a free hormone and bound to plasma proteins
  • Receptor site: surface of target cell
  • Action:            second messenger system
    • Thyroid hormones
  • Structure: iodinated tyrosine derivative
  • Solubility: lipophilic
  • Synthesis: extracellular colloid
  • Storage: extracellular colloid
  • Secretion: endocytosis of colloid
  • Transport: mostly bound to plasma proteins
  • Receptor site: inside of target cell
  • Action: direct effects on genes ® production of new proteins