Purification from Calf Thymus of an Inhibitor of Deoxyribonuclease I

A protein inhibitor specific for pancreatic deoxyribonuclease (DNAase I) was purified 10‐ to 20‐fold from calf thymus. This purification resulted in a nearly homogenous protein demonstrating the relative abundance of the inhibitor among the soluble proteins of the crude extract. Like the DNAase inhibitors from calf spleen described earlier the thymus inhibitor showed a high tendency to form high molecular weight aggregates of low specific activity. Conditions which decreased the rate of this self‐aggregation were found as were conditions for the dissociation of already formed inhibitor aggregates. The molecular weight of the purified inhibitor was determined under both stabilizing and disaggregating conditions and values between 45000 and 50000 were obtained. The interaction between the enzyme and the inhibitor was shown to give rise to a stable complex containing one molecule each of the involved protein species.