Enzyme Purified From Mouse Tumour Cells Reveals Mechanics of DNA Synthesis

Shepherd and Keir reported in The Biochemical Journal in 1966 a biochemistry study describing the purification and characterization of DNA nucleotidyltransferase (a DNA polymerase) isolated from Landschütz ascites-tumour cells, a mouse cancer cell line grown as ascites fluid. This is laboratory enzymology work using cell extracts, not a clinical or animal-treatment study.

The researchers developed a purification procedure that separated the enzyme from contaminating nucleic acid, triphosphatase, and deoxyribonuclease activities. They found the enzyme worked optimally at pH 7.2–7.4, required a DNA primer (with heat-denatured DNA serving three to ten times more efficiently than native DNA), and needed all four deoxyribonucleoside triphosphates for maximal synthesis of polydeoxyribonucleotide — the general biochemical term used here to describe the enzyme's synthesized product, a chain of linked DNA building blocks, not any therapeutic compound.

The reaction depended on a divalent cation, with magnesium far more effective than manganese, while calcium supported no synthesis at all. Sodium and potassium stimulated activity by 25% and 125% respectively, and EDTA boosted the system five- to ten-fold. The enzyme lost over 95% of its activity after one minute at 45°C unless DNA was present, which conferred heat resistance. The synthesized product was degradable by several nucleases, banded at a density similar to the DNA primer in cesium chloride gradients, and showed nearest-neighbour nucleotide patterns consistent with template-directed synthesis.

The study characterizes an enzyme's biochemical mechanism in isolated cell extracts; it says nothing about intact organisms, tissue effects, or any therapeutic application.