Abstract
Films of polyadenylic acid (poly-A) were exposed to liver sections from 4-dimethylaminoazobenzene (DAB)-fed rats in order to determine whether the nucleases acting on these films, like the ribonucleases (RNases), are depressed during carcinogenesis. Normal liver parenchyma gave a positive reaction which was particularly intense in periportal areas. Livers from animals fed the basal control diet showed a similar distribution of enzyme activity but were generally more active than normal livers. In DAB-fed rats, the nodules of hepatic tissue gave intense reactions while the trabeculae of bile ducts and connective tissue, as well as the necrotic areas, were negative. The formation of hyperbasophilic foci at later stages of DAB feeding was accompanied by a loss of enzyme activity. The hepatomas, which apparently derived from such foci, showed weak or negligible activity. Thus the changes in RNases and poly-A hydrolases occur at different stages of the carcinogenic process. The loss of RNase activity precedes the neoplastic transformation while the decrease in the activity of poly-A hydrolases is closely associated with tumor formation, but the induced tumors are deficient in both types of nuclease activity.