Why are restriction enzymes kept in 50% glycerol solution?
To keep them intact...
Restriction Enzymes are used in VERY small quantities, but are usually purchased in slightly larger batches.
If for nothing else, you usually prefer to do different tests with the same batch. The purchased batch therefore needs to be stored for an extended time.
Most enzymes are perfectly happy in their buffer at 4degrees Celsius for a while, but will eventually degrade. 24 hours is usually the accepted limit.
For longer term storage the batch has to be frozen. -20C is the standard, and will keep it for several months. For even longer periods (e.g. one or more years) -70C is the norm.
It is necessary to freeze it quickly, and in small containers (e.g. Eppendorfs) to avoid thawing/refreezing too often.
To avoid damage to the protein at such low temperatures, stabilisers can/should be added, such as DTT (Di-Thio-Treitol), Serum Albumin and, most importantly, Glycerol (5-50%)
Strangely enough, at -20C it is necessary to use 50% Glycerol, whereas at -70 Glycerol is not really necessary, though addition of it can help stabilise the protein....