Hormones

Hormones
Any substance released by a cell and acts on another cell, near or far from site of secretion.
Biosynthetic mechanisms are many. Some protein hormones are synthesized as precursors,which are converted to active form by removal of certain peptide sequences.E.g. Insulin is synthesized as pre-proinsulin (m.wt11500).Removal of some amino acids,peptides produce insulin (m.wt 5734).Thyroxine, a single amino acid hormone. It is synthesized as a glycoprotein precursor called thyroglobulin, which has 115 amino acids.
Other hormones like glucocorticoids/ minerolacorticoids from Adrenal gland are synthesized and secreted in their final active form.
Storage
Hormones are stored in secretory granules within the cytoplasm of endocrine cells. eg. Thyroid hormones are stored in follicles filled with colloid particles. Catechoamines of Adrenal medulla are stored in secretory granules of cytoplasm.
• Storage always protects the molecule from untimely inactivation.
• Steroid hormones are not stored in significant quantities.
• In response to stimulus they are synthesized and released immediately.
Release:
• When the target cells require free hormones, they are released immediately.
• Protein, polypeptide hormones are released by exocytosis or pinocytosis. It involves fusion of granules and cellular membrane, followed by secretion in to blood stream.
• Stimulus excites the endocrine cell.
• The specific enzymes in the storage vesicle activate the hormone before release.
• Disruption of the process by certain drugs interferes with exocytosis.
Transport:
• Some hormones are soluble and do not require transport proteins.
• Free hormone is the fraction available for binding to receptors and therefore represents the active form. Free Hormone concentration correlates best with the clinical status of either excess or deficit hormone.
• Steroid hormones are lipid soluble. They diffuse through cell membrane.
• Specific transport proteins are found in blood for carrying steroid hormones and
thyroxine. Plasma globulins bind to thyroxine, cortisol and sex hormones. The binding is noncovalent type.Some hormones bind loosely to proteins like albumin for transport. Binding to plasma proteins protect them from inactivating systems.
• It also keep the hormones in readily available circulatory form to the target tissues.
Hormones and binding proteins
Hormone Binding proteins.
Thyroxine (T3) Thyroxine binding globulin (TBG), Thyroxine binding
Pre-albumin (TBPA).
Aldosterone Albumin
Estrogen steroid hormone binding globulin (SHBG).
Testosterone SHBG and Albumin.
Cortisol Cortisol binding globulin (CBG).
Control of Hormone Release
Most hormone synthesis and release is regulated through negative feedback mechanisms.
Nervous system modulation allows hormone secretion to be modified by the nervous stimulation in response to changing body needs.
Hormone action and Signal Transduction
Cellular functions are regulated by hormones, neurotransmitters and growth factors through their interaction with the receptors, located at the cell surface. Some hormones elicit hormonal cascade system.
Based on mechanism of action, the hormones may be