The ribonuclease activities of seven ascites tumors of mice have been studied. Homogenates of all tumors exhibited two apparent activity optima—one between pH 4.7 and 5.4, the other between pH 8.5 and 9.3. These two values are outside the range in which normal tissues have optimal activity.

In the presence of p-chloromercuribenzoate, these two optima disappeared and were replaced by a single peak at a pH of 7.0–7.3, suggesting that the two optima seen in the absence of p-chloromercuribenzoate result from the dissociation of an enzyme-inhibitor complex.

A ribonuclease inhibitor has been isolated from Ehrlich ascites tumor cells and shown to inhibit bovine pancreatic ribonuclease. An investigation of the effect of pH on the inhibition of the crystalline enzyme by the tumor cell inhibitor has provided data which support the premise that the two optima obtained with tumor cell homogenates in the normal assay system are due to dissociation of an enzyme-inhibitor complex.


Supported by grants from the National Institutes of Health (C-4534 and 2G-142-C1), the American Cancer Society (E-89), and the Samuel S. Fels Fund.

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