Nucleic acid removal

Intracellular enzyme preparations contain nucleic acids which can give rise to increased viscosity interfering with enzyme purification procedures, in particular ultrafiltration. Some organisms contain sufficient nuclease activity to eliminate this problem but, otherwise, the nucleic acids must be removed by precipitation or degraded by the addition of exogenous nucleases. Ammonium sulphate precipitation (see later) can be effective in removing nucleic acids but will remove some protein at the same time. Various more specific precipitants have been used, usually positively-charged materials which form complexes with the negatively-charged phosphate residues of the nucleic acids. These include, in order of roughly decreasing effectiveness, polyethyleneimine, the cationic detergent cetyltrimethyl ammonium bromide, streptomycin sulphate and protamine sulphate. All of these are expensive and possibly toxic, particularly streptomycin sulphate. Also, they may complex undesirably with certain enzymes. They may be necessary, however, where possible contamination of the enzyme product must be avoided, such as in the preparation of restriction endonucleases. Otherwise, treatment with bovine pancreatic nucleases is probably the most cost-effective method of nucleic acid removal.