Effects of β-aminoethylisothiuronium (AET) on enzymes of animal tissues

Abstract 1. 1. A molar concentration of 3 × 10−3 AET produced no change in the endogenous respiration of rat spleen, thymus gland, and liver slices, while the oxidation of succinate and pyruvate by the latter tissue was markedly depressed. The uptake of pyruvate by thymus gland slices was increased 73% in the presence of 2 × 10−3 M AET. 2. 2. The oxidation of pyruvate plus fumarate by rat liver, kidney, heart, and brain homogenates was inhibited by AET in vitro, while a dose of 300 mg/kg given 30 minutes before sacrifice failed to cause an effect in this system. However, AET at a dosage level of 200 mg/kg reduced the citrate accumulation in rat kidney, liver, and heart following the administration of fluoroacetate. 3. 3. The oxidation of pyruvate by rat liver and kidney homogenates was 50% inhibited by 4.3 × 10−4 M AET and 3.1 × 10−4 M AET, respectively. Heart tissue was less susceptible to inhibition, while brain was completely refractory. α-Ketoglutarate oxidation by rat liver was 50% inhibited by 6.6 × 10−4 M AET. Both enzymes in liver and kidney tissue were markedly inhibited in vivo following sublethal and lethal doses of AET. Maximum inhibition occurred 30 minutes after injection and was completely reversed within 3 hours after a sublethal dose of the compound. Inhibition of α-keto acid oxidation by AET was not produced through an action on DPN, cocarboxylase, or coenzyme A. 4. 4. The development of the inhibition of α-keto acid oxidation in vitro was progressive with incubation time and accompanied oxidation of AET by the tissue. Rat liver and kidney oxidized the greatest amount of AET, while heart muscle and brain were less active. 5. 5. Succinic dehydrogenase, malic dehydrogenase, and cytochrome oxidase activity of rat liver were 50% inhibited by 2 × 10−3 M AET in vitro. Malic dehydrogenase and cytochrome oxidase inhibition was partially reversed during the latter stages of incubation, while depression of succinic dehydrogenase activity remained constant. The malic dehydrogenase and cytochrome oxidase activities of rat liver were not appreciably altered in vivo by AET. A dose of 380 mg/kg of AET produced 24% inhibition of rat liver succinic dehydrogenase activity 30 minutes after injection. 6. 6. DPNH-cytochrome c reductase activity of rat liver mitochondria was inhibited 84% following a 40-minute preincubation with AET at 38°C. Approximately one-third of the inhibition was attributable to an action on cytochrome c while the remaining effect was due to an action on the reductase complex. The presence of tissue was necessary for the effect on cytochrome c. 7. 7. Liver enzymes in the rat which catalyze the oxidation of pyruvate, α-ketoglutarate, succinate, and cytochrome c were more susceptible to inhibition by AET in vitro and in vivo than the same enzymes of the mouse, guinea pig, and dog. 8. 8. Anaerobic glycolysis in rat liver slices was stimulated by AET. This effect was not mediated through an action on the metabolic reactions involved in glucose-6-phosphate metabolism, nor was any effect produced on rat liver phosphorylase or phosphoglucomutase activity. The administration of AET to rats caused increases in blood lactate concentrations in proportion to the dose of the compound employed. 9. 9. AET at concentrations as high as 3 × 10−3 M failed to inhibit the adenosine-triphosphatase activity of several mouse tissues. Furthermore, this concentration of AET produced no inhibition of purified hexokinase and lactic dehydrogenase activity even after preincubation with the enzymes for 40 minutes at 38° C.