Cancer Research VOLUME31 NOVEMBER 1971 NUMBER11 [CANCER RESEARCH 31, 1523-1542, November 1971] A Review of Isozymes in Cancer1 Wayne E. Criss Department of Obstetrics and Gynecology, University of Florida College of Medicine, Gainesville, Florida 32601 TABLE OF CONTENTS postulated role for that particular isozymic system in cellular metabolism. Summary 1523 Introduction 1523 Normal enzyme differentiation 1523 INTRODUCTION Tumor enzyme differentiation 1524 Isozymes 1524 Normal Enzyme Differentiation DNA polymerase 1524 Enzyme differentiation is the process whereby, during the Hexokinase 1525 Fructose 1,6-diphosphatase 1525 development of an organ in an animal, the organ acquires the quantitative and qualitative adult enzyme patterns (122). Key Aldolase 1526 pathway enzymes in several metabolic processes have been Pyruvate kinase 1527 found to undergo enzymatic differentiation. The enzymes Láclatedehydrogenase 1527 Isocitrate dehydrogenase 1527 involved in nitrogen metabolism, and also in urea cycle Malate dehydrogenase 1528 metabolism (180), are tyrosine aminotransferase (123, 151, Glycerol phosphate dehydrogenase 1529 330, 410), tryptophan pyrrolase (261), serine dehydratase Glutaminase 1529 (123, 410), histidine ammonia lyase (11), and aspartate Aspartate aminotransferase 1530 aminotransferase (337, 388). The enzymes involved in nucleic Adenylate kinase 1531 acid metabolism are DNA polymerase (156, 277) and RNase (52). In glycolysis the enzymes are hexokinase-glucokinase Carbamyl phosphate synthetase 1531 Lactose synthetase 1533 (98, 389), galactokinase 30, aldolase (267, 315), pyruvate Discussion 1533 kinase (73, 386), and lactate dehydrogenase (67, 69). In References 1533 mitochondrial oxidation they are NADH oxidase, succinic oxidase, a-glycero-P oxidase, ATPase, cytochrome oxidase, and flavin content (84, 296). In glycogen metabolism the SUMMARY enzymes involved are UDPG pyrophosphorylase and UDPG glucosyltransferase (19). The enzymes involved in glu- Several distinct isozymic systems which were observed to coneogenesis are glucose 6-phosphatase (79), fructose have alterations during differentiation from the normal to the diphosphatase (18), phosphoenolpyruvate carboxylase (21, neoplastic state have been reviewed. Most of these isozymes 413), and pyruvate carboxylase (17). Most of these enzyme are key pathway or regulatory enzymes of intermediary shifts are simple quantitative changes (296, 410) which have metabolism. The isozymic systems which were reviewed been interpreted as the initiation or termination of a metabolic include DNA polymerase, hexokinase-glucokinase, fructose pathway in that tissue. However, some enzymes undergo shifts 1,6-diphosphatase, aldolase, pyruvate kinase, lactate in their isozymic complements (67, 156, 389), thus making dehydrogenase, isocitrate dehydrogenase, malate the overall metabolic interpretation more or less difficult, dehydrogenase, glycerol phosphate dehydrogenase, depending upon how much is known about each isozymic glutaminase, aspartate aminotransferase, adenylate kinase, form. carbamylphosphate synthetase, and lactose synthetase. The With the same type of reasoning, one can assume that the basic isozymic system was outlined, and alterations which disappearance of a key pathway enzyme in neoplastic tissue occurred in the neoplastic state were discussed in terms of the represents a loss of that particular metabolic pathway to the tissue. The question one can ask is, "Do neoplastic tissues have 1This work was supported by the Florida Division of the American a distinctive and unique enzyme pattern just as the normal Cancer Society (Grant F71UF-1). organ systems in an adult animal have distinctive and unique Received April 21, 1971; accepted June 15, 1971. enzyme patterns?" If so, we should be able to examine key NOVEMBER 1971 1523 Downloaded from cancerres.aacrjournals.org on September 28, 2021. © 1971 American Association for Cancer Research. Wayne E. Criss pathway enzymes and establish potentially functional and enzymes which exist as multiple forms and which have been potentially obsolete metabolic routes in tumors. found to undergo changes in cancers. Tumor Enzyme Differentiation ISOZYMES Numerous reviews have been written which reported the measurable levels in neoplastic tissues of nearly all of the DNA Polymerase enzymes involved in glycolysis (47, 149, 192, 287, 397, 399, 405, 406), pentose-P metabolism (149, 192), gluconeogenesis Deoxynucleotidyl transferase (DNA polymerase) activity (47, 192, 397), oxidative phosphorylation (219, 399), urea exists throughout nature in a heterogeneous state (37, 110, cycle metabolism (418), and nucleic acid metabolism (58, 125, 111, 160, 179, 258, 269, 270,421, 425). Purified Escherichia 246, 352, 408). It would appear to be very difficult, if not coli DNA polymerase used native and heat-denatured DNA as impossible, to extrapolate the abnormal enzyme patterns primer and formed a complex with native and heat-denatured found in one type of tumor directly into other types of DNA (34, 160). Calf thymus DNA polymerase was purified as malignant tissues. The patterns may vary within different 2 separate and distinct forms, a replicative deoxynucleotidyl tumor tissues in the same animal species and may also deviate transferase which used only denatured DNA as primer and was in similar tumor types from different animal species. a DNA polymerase and a terminal deoxynucleotidyl Weber (397) has presented a "molecular correlation concept transferase which used short-chain oligodeoxynucleotides or of neoplasia" which was based principally upon the Morris denatured DNA as primer and was a terminal addition enzyme "minimal-deviation" hepatoma system (247, 254—256). This (37-41, 167, 216, 421). The thymus enzymes did not concept is an attempt to make a phenotypic complex with DNA and thus probably differ from E. coli description of the metabolism of a "typical" tumor cell as polymerase. DNA polymerases, as purified from regenerating follows. Patterns of carbohydrate metabolism show a decrease rat liver (248) from nonhistone chromosomal proteins of in gluconeogenic capacity and an increase in glycolytic normal rat liver (141) and from sea urchin embryo (220), capacity in the hepatoma cell. The capacity for the synthesis preferentially used native DNA as primer. Different physical of glycogen and lipid is greatly decreased. The tumor cell can and enzymatic (e.g., substrate specificity) properties were synthesize proteins and nucleic acids at increased rates, while observed with nuclear and mitochondrial DNA polymerases the catabolism of amino acids, proteins, and nucleic acids are from normal rat liver and yeast (157, 162, 242). Therefore, as decreased. Thus, it would appear that the hepatoma cell has a generalization, DNA polymerase activity can be classified streamlined its metabolism in an attempt simply to grow and according to preference for native or denatured substrate. multiply. However, one must continuously keep in mind that With Sephadex gel filtration, DNA polymerase activity from the in vitro measurements of enzymes or enzyme systems, adult rat liver has been separated into 2 fractions (156, upon which most of the above-mentioned reviews and much of 275—277). Peak I had a large molecular weight, was isolated the foundation for the molecular correlation concept are from the postmicrosomal supernatant, and worked best with based, are an indication only of the potential activity of heat-denatured DNA as primer. Peak II was smaller, was specific enzymes and their representative metabolic pathways. concentrated in the free ribosomes, and used mainly native Loss of an enzyme may or may not be indicative of a loss of DNA as primer (26, 156). Peak I was the predominant form of the specific pathway in which the enzyme plays a key role. DNA polymerase in malignant tumors and fetal liver. Peak II Also, the presence or even the increase in the activity of a key was the predominant form in adult and regenerating rat liver enzyme (especially from in vitro assays) may not necessarily (275). indicate an increase in the corresponding pathway. However, 3 forms of DNA polymerase from rat liver and The latter statements are particularly true in the situations hepatomas have been observed with phosphocellulose column in which a key enzyme exists in multiple forms and the various chromatography (156, 270, 275-277). Peaks I and II used forms have different modulators. Decreases in one enzyme heat-denatured DNA, and Peak III accepted native DNA as form may be compensated by increases in another enzyme primer. Normal liver and slow-growing hepatomas had similar form, with the net result being an increase, a decrease, or no levels of Peaks I and II and a much smaller level of Peak III. net change in the total activity of that enzyme system. Regenerating and fetal liver and fast-growing hepatomas had Therefore, an examination of the changes in the activity levels higher levels of Peak II than of Peak I and only a very small of one of the isozymic forms of a key enzyme system, a amount of Peak III. DNA polymerase I remained fairly process which occurs naturally during the phenomena of constant, while the II form increased and the III form enzyme differentiation and which was cited earlier in this decreased with increased tissue growth rates. It would appear review, could yield considerable information about the that the predominant form of normal adult rat liver DNA functioning of specific metabolic pathways in the tumor cell, polymerase acts on native DNA but