Effect of α-casein on DNA adsorption by Andosols and by soil components

Andosols and the soil components (allophanes, humic acids, and goethite) had been autoclaved to destroy the nuclease activity of soil microflora. DNA adsorption by allophanes and Andosols was decreased by increasing the amount of α-casein added to the allophanes and to soils up to casein concentration of 5 mg ml −1 . DNA adsorption by humic acids was significantly increased by increasing the amount of α-casein up to 1.0 mg ml −1 , whereas the addition of 20 mg α-casein ml −1 completely blocked DNA adsorption. These results can explain why the addition of excess skim milk is operationally needed for effective DNA extraction from Andosols. The amount of DNA adsorbed by Andosols treated with dephosphorylated α-casein was significantly higher than that of not treated Andosols ( p < 0.01), thus showing an opposite behavior than α-casein addition, implying that phosphate groups of the protein can contribute to the decrease in the DNA adsorption by soil components. These results explain that the usefulness of skim milk as an additive for the extraction of DNA from Andosols is due to the presence of phosphate groups and hydrophobic components of casein, the main constituent of skim milk.