I am working on an enzyme activity assay using NADPH as a cofactor. The MSDS for NADPH Tetrasodium Salt, tells to store it in a place away from heat and light. Does this include UV light or just visible light? I was told that a pure NADPH stock solution (with pH 7 phosphate buffer as solvent) could be re-used after absorbing UV light at 340 nm, but based on my understanding of the MSDS it seems that shouldn't be the case. So my question is: could I reuse a sample of pure NADPH stock solution that was exposed to UV for my enzyme activity assay?
In my experience, reuse of NADPH stock solutions maintained at pH 7.0 or above is perfectly acceptable. Taking an absorbance spectrum every half-hour or so will probably show that definitively. However, NADPH is VERY unstable in acid, leading to a 'bleaching' of the absorbance at 340nm. Try measuring a rate at pH 6 or below, 340nm, with 100 micromolar NADPH and no enzyme or cosubstrate. There will be a very significant decrease in absorbance with time (ie a 'blank' rate) which will make estimation of enzymic reactions at low pH virtually impossible.