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37.1B: Lipid-Derived, Amino Acid-Derived, and Peptide Hormones

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    13969
  • All hormones in the human body can be divided into lipid-derived, amino acid-derived, and peptide hormones.

    LEARNING OBJECTIVES

    Recognize characteristics associated with lipid-derived, amino acid-derived, and peptide hormones

    KEY TAKEAWAYS

    Key Points

    • Most lipid hormones are steroid hormones, which are usually ketones or alcohols and are insoluble in water.
    • Steroid hormones (ending in ‘-ol’ or ‘-one’) include estradiol, testosterone, aldosterone, and cortisol.
    • The amino acid – derived hormones (ending in ‘-ine’) are derived from tyrosine and tryptophan and include epinephrine and norepinephrine (produced by the adrenal medulla).
    • Amino acid-derived hormones also include thyroxine (produced by the thryoid gland) and melatonin (produced by the pineal gland).
    • Peptide hormones consist of a polypeptide chain; they include molecules such as oxytocin (short polypeptide chain) or growth hormones ( proteins ).
    • Amino acid-derived hormones and protein hormones are water-soluble and insoluble in lipids.

    Key Terms

    • oxytocin: a hormone that stimulates contractions during labor, and then the production of milk
    • epinephrine: (adrenaline) an amino acid-derived hormone secreted by the adrenal gland in response to stress
    • estrogen: any of a group of steroids (lipid-hormones) that are secreted by the ovaries and function as female sex hormones

    Types of Hormones

    Although there are many different hormones in the human body, they can be divided into three classes based on their chemical structure: lipid-derived, amino acid-derived, and peptide hormones (which includes peptides and proteins). One of the key, distinguishing features of lipid-derived hormones is that they can diffuse across plasma membranes whereas the amino acid-derived and peptide hormones cannot.

    Lipid-Derived Hormones (or Lipid-soluble Hormones)

    Most lipid hormones are derived from cholesterol, so they are structurally similar to it. The primary class of lipid hormones in humans is the steroid hormones. Chemically, these hormones are usually ketones or alcohols; their chemical names will end in “-ol” for alcohols or “-one” for ketones. Examples of steroid hormones include estradiol, which is an estrogen, or female sex hormone, and testosterone, which is an androgen, or male sex hormone. These two hormones are released by the female and male reproductive organs, respectively. Other steroid hormones include aldosterone and cortisol, which are released by the adrenal glands along with some other types of androgens. Steroid hormones are insoluble in water; they are carried by transport proteins in blood. As a result, they remain in circulation longer than peptide hormones. For example, cortisol has a half-life of 60 to 90 minutes, whereas epinephrine, an amino acid derived-hormone, has a half-life of approximately one minute.

    image

    Lipid-derived hormones: The structures shown here represent (a) cholesterol, plus the steroid hormones (b) testosterone and (c) estradiol.

    Amino Acid-Derived Hormones

    The amino acid-derived hormones are relatively small molecules derived from the amino acids tyrosine and tryptophan. If a hormone is amino acid-derived, its chemical name will end in “-ine”. Examples of amino acid-derived hormones include epinephrine and norepinephrine, which are synthesized in the medulla of the adrenal glands, and thyroxine, which is produced by the thyroid gland. The pineal gland in the brain makes and secretes melatonin, which regulates sleep cycles.

    image

    Amino acid-derived hormones: (a) The hormone epinephrine, which triggers the fight-or-flight response, is derived from the amino acid tyrosine. (b) The hormone melatonin, which regulates circadian rhythms, is derived from the amino acid tryptophan.

    Peptide Hormones

    The structure of peptide hormones is that of a polypeptide chain (chain of amino acids). The peptide hormones include molecules that are short polypeptide chains, such as antidiuretic hormone and oxytocin produced in the brain and released into the blood in the posterior pituitary gland. This class also includes small proteins, such as growth hormones produced by the pituitary, and large glycoproteins, such as follicle-stimulating hormone produced by the pituitary.

    image

    Peptide hormones: The structures of peptide hormones (a) oxytocin, (b) growth hormone, and (c) follicle-stimulating hormone are shown. These peptide hormones are much larger than those derived from cholesterol or amino acids.

    Secreted peptides, such as insulin, are stored within vesicles in the cells which synthesize them. They are then released in response to stimuli (e.g., as high blood glucose levels in the case of insulin). Amino acid-derived and polypeptide hormones are water-soluble and insoluble in lipids. These hormones cannot pass through plasma membranes of cells; therefore, their receptors are found on the surface of the target cells.

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