Vasopressin composition

The collecting duct system (connecting tubule, initial collecting tubule, cortical collecting duct, and outer and inner medullary collecting ducts) is an area of fine control of ultrafiltrate composition and volume and is where final adjustments in electrolyte composition are ttuide. In addition, vasopressin (also called antidiuretic hormone see Chapter 29) modulates water permeability of this part of the nephron. 

When, after numerous unsuccessful attempts, oxytocin and vasopressin were finally isolated, one of the most fascinating chapters of the history of medicine was started. Most of the original work was done in Du Vigneaud s laboratory between 1932 and 1953. It involved oxytocin and vasopressin purification, investigation of their amino acid composition and sequence, synthesis of the polypeptides, and investigation of the relationship between secondary structure and activity of both these hormones. 

The recent hypothesis of Steble (547) that bovine vasopressin might not contain phenylalanine but instead isoleucine is probably incorrect, for the composition of bovine vasopressin as described by du Vigneaud has been amply confirmed (11). 

The structures of the two peptides are similar, so it might seem surprising that their functions are so different. However, closer inspection shows that there is one small difference and one major difference in their amino acid composition. Both have a neutral, nonpolar side chain at residue 3, but residue 8 in oxytocin is the nonpolar amino acid leucine, whose side chain is a sec-butyl group, but residue 8 in vasopressin is arginine, whose side chain has a positive charge. As a result, the for oxytocin has a very low affinity for vasopressin and the receptor for vasopressin has a very low affinity for oxytocin. Since they bind different receptors, they have different functions. 

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