[maadijk]

After translation, is insulin stored as a precursor or as the active form of insulin? From what I've read, protein precursors are formed because the subsequent molecule can be dangerous but needs to be available fast. I've heard that insulin is stored as proinsulin, but I can't seem to find anything online about it

[Corribus]

Well, you didn't look very hard.

https://en.wikipedia.org/wiki/Proinsulin

[maadijk]

Well, you didn't look very hard.

https://en.wikipedia.org/wiki/Proinsulin

Obviously read it already.

"...It is then transported to the Golgi apparatus where it is packaged into secretory vesicles, and where it is processed by a series of proteases to form mature insulin."

Does this processing happen immediately, so insulin is stored in the vesicles, or does it happen once it is actually required, meaning that it is proinsulin that's stored? 

[Arkcon]

You're asking a very specific question, and you might not find the answer in a textbook, but instead only in someone's very specialized research paper.  Perhaps someone has isolates just the Golgi vesicles, and carefully analyzed relative amounts of proinsulin to insulin, in order to answer this question.

You're asking if this "dangerous" hormone is stored or not.  I can't help but wonder why you think its dangerous.  Its a hormone, it sends a chemical message.  Now, a protease enzyme, that is something we worry about if our digestive cells might end up digesting themselves.  But if insulin leaks out of the vesicle, into the cell ... so what?  Can you describe for us what's likely to happen, and specifically where?

[maadijk]

You're asking a very specific question, and you might not find the answer in a textbook, but instead only in someone's very specialized research paper.  Perhaps someone has isolates just the Golgi vesicles, and carefully analyzed relative amounts of proinsulin to insulin, in order to answer this question.

You're asking if this "dangerous" hormone is stored or not.  I can't help but wonder why you think its dangerous.  Its a hormone, it sends a chemical message.  Now, a protease enzyme, that is something we worry about if our digestive cells might end up digesting themselves.  But if insulin leaks out of the vesicle, into the cell ... so what?  Can you describe for us what's likely to happen, and specifically where?

I think you misunderstood my original post. I'm not saying that insulin is dangerous, I'm saying that precursor proteins (according to wikipedia) are often produced because the subsequent protein is harmful: "Protein precursors are often used by an organism when the subsequent protein is potentially harmful, but needs to be available on short notice and/or in large quantities."

[Arkcon]

OK, but since this is a learning forum, you can still answer my question:  If the vesicles release insulin, in active form, into the cytoplasm of the secretory cells of the Islets of Langerhans, what will be the result?  If the enzyme pepsin is released into the secretory cells of the mammalian stomach, what happens?

[Babcock_Hall]

The first form of insulin that is synthesized is preproinsulin (which is mentioned elsewhere in Wikipedia).  Preproinsulin is converted into proinsulin.  The maturation of insulin is discussed in some biochemistry textbooks.  Which ones have you consulted?

[Babcock_Hall]

Wikipedia's entries are not sufficiently cross-indexed; there is no mention of the C-peptide, which is co-produced when insulin is produced, in the entry on proinsulin.