b) The preprohormone is transformed into a prohormone as it moves through the endoplasmic reticulum and Golgi apparatus. Definition of prohormone:- A prohormone is a sequence of amino acids which lacks the signal sequence. It is still inactive. c) The prohormone is packaged into secretory vesicles in the Golgi apparatus with proteolytic enzymes that cleave the different elements from the peptide sequence to form active hormone and other fragments. B.1.2. Secretion of peptide hormones (Fig. 7-3):- The active peptide hormone and the other fragments are secreted together into the extracellular fluid (this process is called co-secretion) upon reception of the appropriate signal by the parent cell. B.1.3. Transport in the blood:- Peptide hormones, as they are water-soluble, dissolve easily in the extracellular fluid for transport throughout the body. - Their half-life is usually short, in the range of minutes. B.1.4. Cellular mechanisms of actions of peptide hormones (Fig. 7-5):
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