Podophyllotoxin, 10–3M, inhibits the respiration in vitro of rat lymph nodes, thymus, kidney, tumor, spleen, liver, brain, testis, and chicken embryo. Lymph node and spleen respiration are most sensitive, and the degree of inhibition increases with time.

The injection of podophyllotoxin into tumor-bearing mice (20 mg. per kg.) causes a dramatic reduction in the respiration of tumor slices. Within 6 hours, the respiration approaches zero. Inhibition is evident 2 hours after injection of the drug. Spleen respiration is reduced 50 per cent within 6 hours. Kidney and liver respirations remain within normal limits.

Marked reductions in the respiration of spleen, lymph nodes, and thymus glands of normal rats are produced by the injection of 15 mg. per kg. Thymus gland is the most sensitive of these three tissues, and its respiration is reduced 66 per cent 24 hours after injection of the drug.

The injection of 0.8 microgram podophyllotoxin into the yolk sac of chicken eggs bearing 5 day embryos has no effect on the respiration of the embryo within 8 hours, although this is a sufficiently toxic dose to kill 80 per cent of the embryos (within 24 hours).

Kidney respiration in the presence of acetate, glucose, alanine, and glutamate is inhibited to approximately the same degree as in the absence of added substrate. Succinate and pyruvate oxidation by rat kidney slices appear to be less sensitive.

Oxidation of acetate and butyrate by rabbit kidney homogenate is more sensitive to podophyllotoxin than oxidation by rabbit kidney homogenate without added substrate. Glucose oxidation by this preparation is not inhibited by 10–3M podophyllotoxin.

The anaerobic glycolysis of chicken embryo, rat brain, and rat testis is stimulated by 10–5 and 10–6M podophyllotoxin, and is inhibited by 10–3M.

The following enzymes are not inhibited by 10–3M podophyllotoxin: succinoxidase from pigeon breast muscle, choline, xanthine and tyrosine oxidase from rat liver homogenate, and leucine oxidase from Proteus vulgaris; alkaline and acid phosphatase from dog serum; adenosine triphosphatase from rat liver; choline esterase from rat brain homogenate; ribonucleodepolymerase from spleen mince and thymonucleodepolymerase from dog serum.

High concentrations of podophyllotoxin do not influence the viscosity and degree of polymerization of thymonucleic acid.

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