How to do a kinetic assay measuring NADH?

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How to do a kinetic assay measuring NADH?

Hi, i am trying to measure the activity of an enzyme called alanine aminotransferase in plant tissue (sorghum shoots and roots).  The assay i am using monitors NADH oxidation: when enzyme activity is higher, NADH is oxidized, so its concentration in the reaction decreases.  I have done a kinetic assay and obtained the delta absorbance value (dA/min) at 340 nm (5 minutes total, 9 second intervals); my question is: how do i convert the dA/min value to micromol/ microgram protein/ minute???  Do i have to make a NADH standard curve or is there any formula?  I am very confused and would really appreciate if someone could help me.

Thanks in advance,