Well your problem is that uM is a concentration (umol/L) and as such you cannot convert it into nmol alone without knowing the reaction volume. If for example you place your 1 mg of tissue into 1 mL and than measure the concentration you get a concentration for your 1 mL - let's say 2 uM - and since that is from 1 mg tissue you get 2uM/mg. To get to nmol/mg you have to take concentration*volume (2uM*0.001L) to get to moles 0.002umole = 2 nmole and divide the result by the tissue mass (2 nmole/1 mg tissue) = 2nmole/mg.