If you are going for internal differences and not absolute values some degradation could be acceptable. In that case the number of freeze/thaw cycles are more important then storage time. It's impossible to predict the effect of your freeze/thaw cycle since it would be time/temperature dependent but the stability of proinsulin and insulin should be very similar under these conditions.

The only way to predict the stability of your samples would have to be to follow standard QC/QA protocols for long term storage. E.g. a control sample could have been frozen in different batches and then analysed at different time points to measure the integrity of your samples. If the samples are from blood/plasma, optimal storage time would probably be somewhere between 3 to 12 months. Then again, if the quality doesn't have to be 100%, storing it for several years shouldn't be a problem.

We have performed stability tests at different temperature levels and with different numbers of different freeze thaw cycles and different specimens in order to determine the best way to store and maintain our analytical standards. Intact proinsulin is degraded by muscle collagenases and unspecific serum proteases. If kept in an EDTA plasma sample, the molecule is surprisingly stable. In fact, we have kept standard samples for more than 10 years now at -80°C without loosing more than 3-5 % of the originally measured concentration. At -20°C, it is stable for up to 2 years. At 4-8 °C it is 4-8 weeks.

Serum samples are much less stable at 4/8°C and -20°C but at -80°C you can also keep them for up to 3 years. I hope this helps

With respect to freeze thaw cycles: We we performed up to 10 freeze-thaw cycles without substantial impact in EDTA samples stored at -80°C

Tim,

It seems that Andreas has provided you very useful information. 

In my case, I do not recall how many freeze thaw cycles I tested but in the presence of EDTA-Trasylol a few cycles (2-3) were not too bad.

As a general rule, however, I would highly recommend that when you thaw your samples the first time, add EDTA+Trasylol and aliquot them into multiple portions so they sufficient for your single future use. A little extra effort for samples that are precious is worthwhile to maintain the integrity of samples and consequently quality of results, even if it may seem excessively cautionary. 

Good luck!

Hallo; Proinsulin is stable at -20°C for at least 6 months, at lower temperature probably even longer. There exists a publication of the German association of Laboratory Medicine, accepted also by the WHO, which is called "Quality of diagnostic samples", including references (in the case of proinsulin 3 references),and wuich is included. If you want more information, ask me or contact me ([email protected]).

greetings Sighart Walter Golf

Dear Andreas Pfützner ,

I have a question about proinsulin. What concentration of EDTA do you use in the proinsulin stocks?

Best regards,

Alejo