Originally Answered: Clarify enzyme activity (umol/mg/min) these units?
Well, you or your professor certainly have made this confusing but it doesn't need to be. Enzyme activity is generally given as moles/unit time, e.g. per min. The specific activity of the enzyme would incorporate the mg protein of the enzyme. So, you might have something like nmoles/mg/min. It is, of course, possible to use molarity, if you know the volume of the solution in which the enzyme reaction takes place. So, you could then have nM/min or nM/mg/min. Again, the use of mg assumes you know how many mg of enzyme protein you have. Now, as for the double reciprocal plot, you are correct that it plots 1/v as a function of 1/S, and the Y intercept is the reciprocal of the Vmax, given in units of nmole/min, nmole/mg/min, or nM/min or nM/mg/min. Again, using mg assumes you know how many mg of enzyme protein you have. Hope all this now makes more sense. Let me know.