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This textbook says:

In the classic definition, hormones are secretory products of the ductless glands, which are released in catalytic amounts into the blood stream and transported to specific target cells (or organs), where they elicit physiologic, morphologic and biochemical responses.

What is meant by catalytic amount? Does it mean that amount required for catalysis? I know only renin which is catalytic and act as an enzyme, and many other hormones are non enzymatic so why the term catalytic is used in general sense?

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  • $\begingroup$ In my opinion it isn't a very good phrase for a textbook because it doesn't have much meaning, but I believe what they are trying to convey is that they are released in small quantities, like is typical of enzymes - you don't need a lot because they work over and over again. By contrast, there are a lot of proteins in the blood that are expressed in fairly high quantities, like those involved in coagulation, to the point that they make up a non-negligible component of the blood. $\endgroup$
    – Bryan Krause ♦
    Mar 9 '17 at 4:07
  • $\begingroup$ @BryanKrause - I think they mean simply enough hormone to get the desired response. It can be alot or a little. $\endgroup$
    – anongoodnurse
    Mar 9 '17 at 5:40
  • $\begingroup$ @anongoodnurse Perhaps, I guess that's why it really isn't clear. When a recipe for a chemical reaction says "catalytic amount" of something they usually mean a very small amount compared to the actual amount of reactants...like a "catalytic amount of platinum" - it just seems really strange to apply that terminology to hormones. This looks like one of the English textbooks written by a non-native speaker that we see a lot of questions about - sometimes the answer is just that a non-native speaker used a phrase they learned in a different context in a flawed way. $\endgroup$
    – Bryan Krause ♦
    Mar 9 '17 at 5:43
  • $\begingroup$ @BryanKrause - I too think it odd phrasing and unclear. I think of a catalyst as something that facilitates a chemical reaction. A "catalytic amount" of insulin depends on how many glucose molecules are in the bloodstream... $\endgroup$
    – anongoodnurse
    Mar 9 '17 at 5:52
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The writer obviously means “small amount”, and has chosen his words badly as hormones do not themselves catalyse reactions.

The idea he is probably trying to convey is that a few molecules of hormone binding to their receptor can cause changes to many molecules within the target cell. This often (generally?) involves catalysis within the cell produced by the receptor activating an enzyme, so one can understand the line of thought leading to the misuse of this term.

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