An improved filter-disc assay for 3H-DNA and serum deoxyribonucleases

1. The filter-paper disc assay for tritium-labeled nucleic acids was modified to reduce washing and manipulation. Evaluation of the procedure indicated greatly enhanced counting efficiency for tritium, complete removal of nuclease digestion products and retention of even nanogram levels of radioactive DNA on the discs in the presence of protein. The modified assay was quite sensitive and had a coefficient of variation of 5.2%. Quite low concentrations of DNase I and endogenous human serum nuclease activity were readily measured by the improved technique, 22 sera yielding a mean value equivalent to about 0.5 ng DNase I/ml serum. We are now using the disc assay to follow serum DNA and DNase levels in patients with autoimmune disorders and various types of cancers.