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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?

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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.

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