| | Re: GST enzyme unit
Let's say that 25 ul of the enzyme preparation (pure or not pure) gives an initial rate of 90 pmol/min under defined assay conditions (constant pH, Temp, etc).
Let's also assume that the protein concentration of the enzyme preparation, which you have determined separately by the Lowry method or the Markwell method or the like, is 10 mg/ml (10 milligrams per ml).
We can then write:
1. Activity of preparation = 90 pmol/min/25 ul.
2. As 25 ul (25 microlitres) is equivalent to 250 ug (250 micrograms) [from the protein concentration measurement]
3. Specific Activity of Preparation = 90 pmol/min/ 250 ug
or, converting to 'per milligram' basis.
4. Specific Activity of Preparation = 360 pmol/min/mg.
Twomportant assumptions of such calculations, which you probably know, are:
(i) The velocity versus (amount of enzyme added) plot should be linear and through the origin.
If, say, 25 ul gives a rate of 90 pmol/min then 50 ul should give a rate of 180 pmol/min and 12.5 ul should give a rate of 45 pmol/min., etc. Furthermore, if there is a 'blank' rate, this must be subtracted.
(ii) You are measuring the *initial* rate.
Two great references:
1. I. H. Segel "Biochemical Calculations (3rd Edition)
2. Enzymes by Malcolm Dixon, E.C Webb, K.F. Tipton & C.J.R Thorne (3rd Edn, 1979). (In particular the first couple of chapters)
I hope this is of help, and is what you want.
If not you can let me know, perhaps by posting a specific example? In my view, specific activity measurements are important. 'Specific', in the sense used, means express on a *per weight* basis.