Practical Considerations for Enzymatic Assays

Enzyme activity is preserved by cold storage. Lyophilized (freeze-dried) proteins are stored in a freezer or refrigerator, while dilute solutions are stored at 2-5 °C. Concentrated suspensions of enzymes in ammonium sulfate are usually stable for long periods at 2-5 °C. 

Some enzymes strongly adsorb to glassware. If this is the case, solutions are commonly prepared in the presence of a large (100-fold) excess of an inert protein, such as albumin, to avoid activity losses that would lead to miscalibrations. 

The reduced forms of the pyridine coenzymes, NADH and NADPH, are sensitive to light (20% loss over 7 weeks), moisture (50% loss in 24 h), and high temperatures (10% loss over 3 weeks at 33 °C). The oxidized forms of these coenzymes are more stable, but should also be stored at low temperature. Flavin cofactors are light sensitive, and should be stored in the dark. 

Buffered solutions should be prepared under sterile conditions, and stored after ultrafiltration. Many biochemical buffer systems are excellent growth media for bacteria. 

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