A typical unit for enzyme specific activity is µmole/min/mg, but smaller units might be used for enzymes with very low rates of catalysis, or very crude preparations with little enzyme in them. For example, nmol/g/h is 60,000,000 times smaller a unit of activity than µmol/min/mg. Enzyme catalytic rates vary over many orders of magnitude. It's possible that the preparations of the enzymes you mentioned (BG=beta-galactosidase, CBH=cellobiohydrolase, PPO=polyphenol oxidase, PER=?) all have about the same specific activity, but it would be a remarkable coincidence.

Enzymes that are well known (collagenase, pancreatin, etc.) can safely be described in mg/ml or ug/ml terms since they are often used in methods to prepare something and are often supplied in excess.  For PCR or more specific applications, you might list an enzyme in Units.  The reason we always list the supplier is so that someone can go to the website and, if they want to, work out how many Units of Enzyme X turns over ___ ug of substrate in ____ time period.   Until everyone starts using the katal as intended, there will always be confusion.

What this means is that each lab tends to report enzyme units in their own way and someone has already figured out the math.  If it's incompatible with your own lab's units of preference, it can be an uncomfortable conversion. 

This website that I attached might help:

http://www.physiologyweb.com/calculators/units_per_volume_solution_concentration_calculator.html

1 unit (U) is the amount of enzyme that catalyses the reaction of 1 µmol of substrate per

minute or 1 unit (U) is the amount of enzyme that catalyses the reaction of 1 nmol of substrate per minute. Both defination are correct. 1µmol = 1000nmol and with this you can interchange the units.

Hi there,

U (1µmol/min) is actually the most usual unit for describing enzyme activity but it is not actually the SI one as µmol and minute are not SI units. The SI unit for activity is katal which is the amount of material required for transforming 1 mol of substrate per seconde. So 1 U is 16,67 nanokatal...

In my experience, the enzymatic activity is normally described as µmol/min. In certain cases, i.e. the numbers are too small, nmol/min could also be used. 

micromole/min/mg  is the right unit for activity of enzyme.

I enjoy learning from these exchanges in Research Gate. Despite the fact that the katal has been around for a long time, apparently (according to Wikipedia), and I have been working with enzymes for decades, this is the 1st time I have ever seen it mentioned. Interesting.

Thanks guys for the informative inputs. The katal is also the first time I learn from answers to this question above. 

I agree with you Adam: the katal is never used (I only met it once so far and it was in an exercise book, not in a lab) and typically useless for the study of enzyme just because it is too huge (one katal is 6.107U)!!!

Dear colleagues I have to remind an important point is that we have to distinguish between "Enzyme activty" and "Specific enzyme activity". In the first case it is the speed of the enzyme to catalyze the reaction under working conditions so the unit in most cases is µmol/min. The specific activity is the enzyme activity divided by enzyme amount used in the reaction (in general we use mg) so that the whole unit for "Specific Enzyme activity' is µmol/min/mg. Hope this information will be helpfull.

Hallo!

I really wonder about this discussion. The enzyme activity must be expressed by the unit katal, at least in Germany. Since 2009 the kat (1mol substrate metabolized/sec) is the official unit for enzyme activities (Dritte Verordnung zur Änderung der Einheitenverordnung vom 25. September 2009 (BGBl. I S. 3169) and Nomenclature Committee of the International Union of Biochemistry (NC-IUB) (1979). "Units of Enzyme Activity". Eur. J. Biochem. 97 (2): 319–20. doi:10.1111/j.1432-1033.1979.tb13116.x.). Despite this "law", enzyme activities are still often expressed in units, for example in medical diagnosis, since medical doctors are very conservative.

However, if you publish in scientific papers, you usually have to use SI-Units, i.e. the kat.

µmole/unit time/unit weight can be used

is U/min/g FW =U/mg protein

Colleagues:

I am a retired Professor of Food Microbiology, who worked all my professional life (~48 years) in bioprocessing of some of the Industrial enzymes, specifically, alpha amylase, beta-amylase, arabinosidase, beta-glucosidase, cellulases, collagenase, esterase, glucose isomerase, glucosidase, keratinase, ligninase, lipase, pectinase, acidic and alkaline proteases, pullulanase, xylanase, xylitol dehydrogenase, xylose reductase, xylosidase among others. I would agree with A. Shapiro that this is the first time, I learnt about the katal as a unit for enzyme activity. I would also agree with Dr. Boullagui that units of enzymes are defined as Unit or specific units, wherein specific unit takes in to consideration the amount of protein added to catalyze a reaction. Dr. Mitra's definition is all-encompassing, yet the well-known definition. Katal is definitely the first mention, I have seen. Never too late to learn.

Sighart Walter Golf is right, the official unit of enzyme activity in the SI system is the katal (mol/s). See the enzymology page of the IUBMB (https://www.qmul.ac.uk/sbcs/iubmb/) as official resource valid internationally, not only in Germany. As Dominique Liger pointed out, the katal is way too large for everyday use, so you are more likely to encounter nkat or fkat.

Why was the change made from the old enzyme unit (U = µmol/min = 16.7 nkat) to the katal? The SI tries to work directly with the basic units (m, kg, s, K, A, mol, candela) and to eliminate all conversion factors that are not equal to 1. In this case, the 60 s/min are eliminated by the change. Such factors are easily forgotten, leading to errors that can be costly (see https://www.simscale.com/blog/2017/12/nasa-mars-climate-orbiter-metric/ for cases in point). Therefore, I advise my students to work exclusively with SI-units.

Hello, micromole/min/mg

Micromole of product formed /per minute by action of mg of protein in reaction mixture.