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I’m reading on hormones and the book talks about how peptide or amine hormones are easily broken down by proteases present in the blood plasma. This has led me to question the interactions between these proteases and the blood proteins of the blood (such as albumins and globulins). Does having proteases in the blood mean the blood proteins are constantly broken down? How do the blood proteins get anything done then? And what good do binding proteins do in protecting hormones during vascular transit when the binding proteins are also susceptible to blood proteases?

Edit: I’ve been requested to provide some quotes and references.

Because water-soluble hormones can dissolve in blood, many circulate as free hormones, meaning that most of them dissolve directly into the blood and are delivered to their target tissue without binding to a binding protein.

The book goes on to say

Water-soluble hormones, such as proteins, peptides, and amino acid derivatives, have relatively short half-lives because they are rapidly degraded by enzymes, called proteases, within the bloodstream. The kidneys then remove the hormone breakdown products from the blood.

These quotes were from Seeley’s Anatomy and Physiology, 10th edition.

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    $\begingroup$ When asking a question like this, it is usually best to cite the source that gave rise to the question. Also making a direct quote and attributing it to the reference makes it easier to determine the context of the assertion. $\endgroup$ – AMR Nov 7 '15 at 13:15
  • $\begingroup$ @AMR I have added some. $\endgroup$ – lightweaver Nov 7 '15 at 13:29
  • $\begingroup$ Basically it is that you are dealing with textbook vague. They don't define for you what relatively short half-lives is or the enzymes that are in circulation. IgG, which is the major immunoglobulin circulating in the blood has a half-live of between 11-21 days, depending on its subclass. Also the proteases may have certain specificities but not others. Certain proteins may not contain the moieties that the enzymes need to bind and catalyze the reactions and this acts to "protect" the protein from degradation. Without them providing the specifics of what they are referring to, its hard to say. $\endgroup$ – AMR Nov 7 '15 at 13:50
  • $\begingroup$ So is 11-21 days relatively short in the authors' estimations or are they referring to half-lifes on the order of minutes to hours? Unless they define it for you, it is difficult to say. $\endgroup$ – AMR Nov 7 '15 at 13:53
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    $\begingroup$ I think that you also have to remember that hormones are generally signaling molecules that induce transcriptional activity in the cells that receive them. They tend to be released based on certain stimuli, and most of the time, you only want them on for short periods of time, i.e., different stages of development, fight or flight, respond to changes that cause an imbalance in homeostasis, repair damages to cells and tissues. So you would want enzymes that can quickly degrade these molecules, in matters of minutes or hours, so that you don't over-respond to the stimulus. $\endgroup$ – AMR Nov 8 '15 at 1:49

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