As with purine analogues,188 184 anti-pyrimidines, such as 5-fluorouracil, are degraded by the neoplastic tissue much less readily than by normal tissues.130

Thus a reasonable case can be made out in favor of deficiency of catabolic enzymes of nucleic acid metabolism. Yet there are indications that corresponding anabolically acting enzymes might be increased, a state of affairs which would contribute to an even greater imbalance between the degrading and building-up processes in tumor cells. These apparent increases may be due, as Potter199 explained, to a lowering of the level of negative feedback on enzyme formation (see Fig. 4) and could be also explained by Hinshelwood's131 enzyme adaptation scheme in bacteria, mentioned before. While Bollum and Potter40 found an increase of what they called "DNA synthetase" in regenerating rat liver (whether this corresponds to Romberg's DNA-synthesising enzyme (see Fig. 6) has not yet been established), Auerbach and Waisman 16 and Calva et a/.8304 in agreement with Le Page and Sartorelli171 demonstrated an increased de novo synthesis of pyrimidines in tumors.211 Furthermore Reichard and Skold,212 whose findings on catabolic deficiencies have been mentioned above, reported considerably higher levels of the synthesising enzymes uridine phosphorylase and kinase in their ascites material.

Fig. 11. Pyrimidine Transformations (R=ribosido-, DR = deoxyribosido-, RP = ribosidophosphate-, DRP = deoxyribosidophosphate- Residues,↑ Cata-bolism, ↓ Anabolism, ↓↑ Uncertain recations in vivo).

At this point short descriptions of the behaviour of the nu-cleolytic enzymes RNase and DNase are appropriate, the former degrading RNA by a phosphate transfer reaction and hydrolysis but also possessing certain synthesising functions. According to recent literature there exist several RNases which differ either in their pH optima (alkaline, acid and maybe neutral), their tissue origin (pancreas, liver, etc.) or their nucleotide end-products.270 Even crystalline pancreatic RNase has been shown to be heterogeneous and samples from different manufacturing sources seemingly exert different biological effects.182 Moreover such crystalline preparations may contain variable amounts of acid and alkaline enzyme. One gains the impression from some publications (see references 153, 168) that there may be a lower level of acid RNase, at least in some tumors, such as hepatoma, ascites and carcinoma of the human cervix.163 However, this subject requires further studies particularly with respect to the role of acid and alkaline RNase and with reference to the base-line used for calculation of the enzyme activities. Sometimes these activities are presented per unit DNA, sometimes per nucleus and thus per cell. It should be pointed out here again 92 that, when analysing biological material for the content of cellular constituents or levels of enzymic activity, the choice of wet weight, dry weight, nitrogen, phosphorus, protein, DNA or nuclei as baselines for calculation of data may be a critical one and could lead to considerable misunderstandings when comparing one set of figures with another from a different laboratory. Moreover, the heterogeneity of the tissues or cells under investigation, consisting in many cases of variable amounts of connective tissue elements, necrotic areas and different kinds of functional cells, could lead to faulty conclusions.

All this, it is thought, has been demonstrated with the DNA depolymerising enzyme DNase which Brody and Balis56 found to be relatively low in malignant human and animal tissues, a fact which tallies with the increased rates of DNA synthesis during cell division. On the other hand, when de Lamirande, Allard and Cantero154 compared the levels of this enzyme in normal rat liver, hepatoma and liver of animals fed a carcinogenic dye, they arrived at different values which depended on their calculation per gram tissue, per mg. of nitrogen or per average cell. Daoust and Cantero79 on reinvestigating similar materials and starting from the assumption that such data may represent variations in the cytological composition of the tissue, variations in the enzyme activity of the cells or variations in the enzyme activity of extra-cellular fluids, applied histochemical methods to normal, precancerous, cirrhotic and neoplastic livers. While the DNase activity as measured by biochemical analyses showed per cell relatively little differences, histochemical analyses disclosed considerable ones (Fig. 12).

From this single example it can be seen that our present-day knowledge of enzyme levels, belonging not only to the RNA-DNA , group, has to be re-examined and greatly extended, unless very low or zero levels of enzyme activities have been encountered. But even then it will be necessary to ascertain whether this apparent absence or near-absence of activity is due, as mentioned before, to temporary deactivation of enzymes without their disappearance, to a deficiency in apoenzyme or its enzyme forming system or to a lack of co-factor or factors.

Fig. 12. DNase activity of normal, cirrhotic and neoplastic rat livers. (After Daoust and Cantero: J. Histochem. Cytochem." Courtesy of the Williams and Wilkins Company, Baltimore, Maryland.).