Dear all,

can somebody please help me with calculation of enzymatic activity?

I am measuring ATPase activity with an assay where the hydrolysis of ATP is coupled with the oxidation of NADH. The reaction rate is determined by measuring the decrease in absorbance at 340 nm resulting from the NADH oxidation. I think I need to use Beer-Lambert law to calculate specific activity of my ATPase (=ATP consumption=c) in mM/min/mg. I have following information: change of absorbance over 1 min (ΔA=0.2), pathlenght of a well (l=0.3cm), extinction coefficient of NADH (ε=6220 M-1 cm-1) and total amount of the enzyme added to well (0.2mg). I would calculate the total activity like this: c= ΔA/ ε*l. Therefore, in this case c=1.07*10-4. Then I would divide c by total amount of enzyme, thus final specific activity = 5.36*10-4. I am not sure if this correct and also I don’t know what my final units will be in this case? Is it mM/min/mg or M/min/mg because the ε is in M?

Thank you very much for help. My biochemical skills are horribly rusty!