7.5: Types of Hormones

Amine Hormones

One subset of hormones, termed amine hormones, originates from the modification of amino acids. In this transformation, the carboxylic group (–\(\ce{COOH}\)) of the amino acid is excised while the amino group (–\(\ce{NH2}\)) persists. Notably, these hormones find their genesis in amino acids such as tryptophan and tyrosine. A prominent example is melatonin, secreted by the pineal gland, governing circadian rhythms. Tyrosine derivatives encompass thyroid hormones (thyroxine or T4 and triiodothyronine or T3), regulators of metabolism, and catecholamines (epinephrine, norepinephrine, and dopamine). Epinephrine and norepinephrine, secreted by the adrenal medulla, are key players in the fight-or-flight response (see Chapter 5). Dopamine, released by the hypothalamus, inhibits prolactin release (prolactin-inhibiting hormone) by the anterior pituitary.

Peptide and Protein Hormones

Peptide, polypeptide, and protein hormones constitute the second classification, distinguished by their amino acid composition. Peptide hormones comprise short amino acid chains, typically containing around ten amino acids. Polypeptide hormones extend this chain, incorporating up to 100 amino acids. In contrast, protein hormones are more extensive and may consist of multiple polypeptide chains. Notably, insulin is constituted by two polypeptide chains. Some examples of peptide hormones include antidiuretic hormone (ADH), a pituitary hormone governing fluid balance, and atrial-natriuretic peptide (ANP), synthesized by the heart to regulate blood pressure. The gonadotropin-releasing hormone, produced by the hypothalamus, is a polypeptide hormone. Protein hormones encompass growth hormone, produced by the anterior pituitary, and follicle-stimulating hormone (FSH), classified as a glycoprotein due to its carbohydrate moiety. Amino acid-derived hormones can be stored within cytoplasmic vesicles of endocrine cells and released upon appropriate stimulation, contrasting with steroid hormones that are synthesized on demand.

The synthesis of peptide, polypeptide, and protein hormones parallels the fundamental process of protein synthesis. DNA undergoes transcription, yielding mRNA, which is subsequently translated into an amino acid chain. Initially, peptide, polypeptide, and protein hormones are synthesized as preprohormones, which contain a signal peptide necessary for directing the nascent protein to the endoplasmic reticulum. After cleavage of the signal peptide, the preprohormone is converted into a prohormone, an inactive intermediate. Further post-translational processing, including cleavage of specific peptide sequences, results in the formation of the mature, biologically active hormone (Figure 6). This sequential processing ensures proper folding, modification, and activation of the hormone before it is secreted into the bloodstream to exert its physiological effects . For example, insulin traces its lineage back to preproinsulin.

Steroid Hormones

Steroid hormones constitute the lipid-derived category, utilizing cholesterol as a precursor in their synthesis. Reproductive hormones like testosterone and estrogens, originating from gonads (testes and ovaries), belong to this group. The adrenal glands contribute aldosterone, a hormone central to osmoregulation, and cortisol, a key player in metabolism, to the lipid-derived category. These hormones exhibit low solubility in water and necessitate transport proteins or carrier proteins for circulation to their target cells. This intricate structure significantly prolongs the half-life of steroid hormones compared to amino acid-derived hormones. The half-life denotes the time required for half of the hormone concentration to degrade. For instance, cortisol boasts a half-life of approximately 60 to 95 minutes (Table 4), whereas epinephrine, an amino acid-derived hormone, lingers for about one minute.