Attempts have been made to prepare rat liver microsomes and ribosomes free of RNase activity. Washing of microsomes with a large number of reagents, as well as preparation of microsomes by homogenizing the liver in the presence of a variety of reagents chosen to remove or inhibit RNase activity, failed to abolish completely the enzyme activity. However, when rat liver was homogenized in the presence of optimal concentrations of ATP the microsomes subsequently obtained showed no RNase activity. The composition of such microsomes was compared to controls prepared without the use of ATP. Preparation of microsomes with the use of ATP apparently repressed but did not remove the RNase activity for, when such microsomes were treated with 1 per cent deoxycholate to obtain ribosomes, the latter exhibited normal RNase activity. A possible explanation for these results based on several experiments is given. The incorporation of C14 of L-leucine-C14 into control and ATP-treated microsomes was measured. Repression of RNase activity by use of ATP or with RNase inhibitor, significantly reduced the incorporation. As a result of these and other experiments it is tentatively concluded that an alkaline RNase is a normal constituent of rat liver ribosomes and plays a role in the biological activity of these particles.

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