DETERMINATION OF PROTEIN ADMIXTURE IN HIGHLY PURIFIED DNA PREPARATIONS

A version of the Lowry method was proposed for determining the protein content in DNA preparations, whish allows one to measure the color re action of the purest of the samples tested, corresponding to 0.3-0.4% protein. Independent methods were applied to show that the contributio n of mammalian DNA to the color reaction (C-DNA) corresponds to 0.3% o f protein admixture, i.e., the preparations examined actually containe d less than 0.1% protein. With the Lowry method, the protein content i n DNA is overestimated by 0.007 . X + 0.03, where X is the percentage of GC pairs in DNA. The protein content may also be assessed from the spectra of concentrated DNA solutions at lambda 320 nm, e.g. as C-prot ein(%) = 11 .(D-320/P-0.01), where D-320 is the optical density of-the DNA solution at 320 nm, and P is the DNA concentration (mg/ml).