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J Biol Chem, Vol. 275, Issue 14, 9893-9896, April 7, 2000

ACCELERATED PUBLICATION
A Ribonuclease A Variant with Low Catalytic Activity but High Cytotoxicity^*

Lynn E. Bretscher^§, Richele L. Abel^¶, and Ronald T. Raines^**

From the  Department of Biochemistry and the  Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706

Onconase^TM, a homolog of ribonuclease A (RNase A) with low ribonucleolytic activity, is cytotoxic and has efficacy as a cancer chemotherapeutic. Here variants of RNase A were used to probe the interplay between ribonucleolytic activity and evasion of the cytosolic ribonuclease inhibitor protein (RI) in the cytotoxicity of ribonucleases. K41R/G88R RNase A is a less active catalyst than G88R RNase A but, surprisingly, is more cytotoxic. Like Onconase^TM, the K41R/G88R variant has a low affinity for RI, which apparently compensates for its low ribonucleolytic activity. In contrast, K41A/G88R RNase A, which has the same affinity for RI as does the K41R/G88R variant, is not cytotoxic. The nontoxic K41A/G88R variant is a much less active catalyst than is the toxic K41R/G88R variant. These data indicate that maintaining sufficient ribonucleolytic activity in the presence of RI is a requirement for a homolog or variant of RNase A to be cytotoxic. This principle can guide the design of new chemotherapeutics based on homologs and variants of RNase A.

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^* This work was supported in part by National Institutes of Health Grant CA73808 (to R. T. R.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^§ Present address: Dept. of Biochemistry, Medical College of Wisconsin, Milwaukee, Wisconsin 53226.

^¶ Supported by Molecular Biophysics Training Grant GM08293 from the National Institutes of Health.

^** To whom correspondence should be addressed. Tel.: 608-262-8588; Fax: 608-262-3453; E-mail: raines@biochem.wisc.edu.

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Copyright © 2000 by The American Society for Biochemistry and Molecular Biology, Inc.

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