[pofulover]

Why does NADH and NADPH have the same molar absorptivity?

I know it has something to do with structure in terms of adding an enzyme. For example, if I were to add an enzyme it would make the ring incorporate a H, but then does that mean the molar absorptivity has something to do with the enzyme making the spectrometer able to get a wavelength, since an enzyme adds the proton on to the compound.

[Arkcon]

Why does NADH and NADPH have the same molar absorptivity?

I was not aware that they did.  If you have a reference, then fine, but I would expect the same molecule, nicotinamide adenine dinucleotide, to have at least a slightly different extinction coefficient, when it has been protonated and phosphorlated (NADPH).

I know it has something to do with structure in terms of adding an enzyme.

NAD is a co-enzyme, not an enzyme.  When complexed with one of the many enzymes that it works with, it is incorrect to call it NAD or NADPH, but instead it should be called an NAD-enzyme complex.  Bound to a large protein molecule, it would certainly have a large change in extinction coefficient.

For example, if I were to add an enzyme it would make the ring incorporate a H, but then does that mean the molar absorptivity has something to do with the enzyme making the spectrometer able to get a wavelength, since an enzyme adds the proton on to the compound.

This bit here is unclear.  Bits of it are correct, but other bits are wrong, or at least, do not follow logically from one to the other.  Maybe you can say it a different way.

A quick Google search leads me to this year old Yahoo question, where someone responds to the exact same question, with a cut-n-paste from a technical journal, that really does nothing to answer the question.  Could this be the source of the confusion?

http://answers.yahoo.com/question/index?qid=20080306164719AAv960B