Metabolism and intracellular processing of protein hormones

This chapter discusses the various aspects of metabolism of protein hormones. It describes how these regulatory molecules are synthesized and stored as precursor molecules and proteolytically processed to generate biologically active hormones. The mechanisms involved in secretion and degradation are also described. The synthesis of the protein hormones is directed by one or more genes–with one or more genes coding for the amino-acid sequence and other genes being responsible for alterations of the peptide to its final form. The biosynthesis of the protein hormones involves the synthesis of molecular forms larger than the polypeptide secreted from the cell. Proteolytic processing of precursor-protein hormones is a widely used mechanism for producing biologically active hormones, with the exception of a few hormones such as growth hormone and prolactin. Proteolytic cleavage of prohormones begins only after synthesis and folding of the prohormone-peptide chain is completed in the endoplasmic reticulum (ER) and the peptides are transferred to and concentrated in the Golgi area. Protein hormones are synthesized by the ribosomes in the form of a preprohormone, which is biologically inactive.