International Unit (IU) conversion in enzyme kinetics

[vogelstrauss]

Dear all,

 

does anybody know how to convert an enzyme activity of e.g. 5.1 IU/mg to a reaction rate, let's say in micromol/s ?

I read that the IU unit for biological activity and it's definition differs from substance to substance. Does anybody have a guess how I can get information of how to convert this for my specific enzyme (creatine kinase)?

 

Thanks in advance,

 

cheers

 

Hans

[CaptainPanic]

No.

I apologise for ranting without giving a definite answer, and I kindly request people who have an exact explanation to give it (and ignore me)... It's just frustration from my side. I have to work with this, and I have studied it in the past...

 

To my knowledge, there are no standards for enzyme activity. There exist many definitions of units, and I believe that the most accepted unit, which is conveniently called "international unit", is denoted with the letter U (not: IU).

 

The IU should not be confused with the enzyme unit, also known as the International unit of enzyme activity and abbreviated as U.(source)

 

The most likely translation of the U is [math]\mu mol ([/math]substrate consumed[math]) / min[/math], which would mean you have 5.1/60=0.085 µmol/(mg*s).

 

But it can be something else... It can be product based (not reactant), it can be in /s (not /min) and whatever you're able to come up with. As an illustration, check out the unit "FPU", or "filter paper unit". This is also a "unit", and also for enzyme activity. And despite the fact that this unit has a different name than the other unit, it's still not clear what it means:

One unit of filter paper (FPU) activity was defined as the amount of enzyme releasing 1 µmole of reducing sugar from filter paper per ml per min.(source)

(Reducing sugar?)

Because cellulase activity in whole broths is nonlinear in regarding enzyme concentration, this atypical assay dictates dilution of the cellulase preparation to a point where 2.0 mg of reducing sugar equivalents is released in 1 h at 50°C and pH 4.8. This amount of enzyme is defined as one filter paper unit (FPU).(source)

Is that the same FPU as the first one? I doubt it.

 

With bio-units, you never know... apologies for subjective remarks in a scientific thread.

 

I have spent days, perhaps weeks, investigating simple things like this... and I'm sure that worldwide many people are suffering the same fate as vogelstrauss and me.

But still nobody came up with the brilliant idea to actually just express enzyme activity in: [math]mol/g([/math]enzyme[math])s[/math]

 

Anyway, the bottom line for me is that as a chemical engineer, I cannot work with publications from certain people because I don't understand it. However, when I work with people I know, then I can ask for a clarification, and without a problem, the activity is converted into SI - if it's so easy, why is SI not the standard??

 

[/rant without answer]

Edited April 23, 2009 by CaptainPanic

[vogelstrauss]

Dear CaptainPanic,

 

The IU should not be confused with the enzyme unit, also known as the International unit of enzyme activity and abbreviated as U.(source)

 

That's it -- I found in another publication of the same authors that they define [math]IU[/math] as [math]\mu mol/min[/math], so I think they used it wrong and should have written [math]5.1 U/mg[/math] instead of [math]5.1 IU/mg[/math].

 

With bio-units, you never know... apologies for subjective remarks in a scientific thread.

I get what you mean... It often takes me about a day to dig out only one kinetic parameter from scientific literature...

 

Thanks a lot!!