Enhancement of ribonucleotide reductase activity at low enzyme concentrations by polyethylene glycol

The estimation of ribonucleotide reductase in cell extracts has been problematical on account of abnormally low activities at low enzyme concentrations, presumably due to subunit dissociation. This problem can be alleviated by assaying the enzyme in the presence of polyethylene glycol. The presence of 15% polyethylene glycol during the assay greatly stimulated ribonucleotide reductase activity at low enzyme concentrations and allowed measurement of enzyme activity in as little as 10(5) mouse L929 cells, a 30-fold enhancement of assay sensitivity. Enzyme activity measured in the presence of 15% polyethylene glycol was proportional to enzyme concentration, thus making possible the accurate measurement of very low levels of ribonucleotide reductase.