Abstract
Nuclear fractions centrifugally separated from rat liver homogenates in isotonic sucrose or in water, containing in each case 1.8 mM CaCl2, and washed repeatedly in the same media, retained a considerable amount of acid deoxyribonuclease activity. Similarly prepared nuclear fractions using calcium-free media lost nearly all of their deoxyribonuclease activity. This retention of activity in the presence of calcium was shown to be neither cosedimentation of unbroken cells or of lysosomes with the nuclear fraction nor precipitation of soluble deoxyribonuclease in the presence of calcium. The nuclear-bound enzyme was active under conditions shown not to remove it from the nuclei.