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91.
The hypothesis that ambient CO2 levels determine the end-products of energy metabolism excreted by Hymenolepis diminuta was tested by incubating the parasite in a range of CO2 concentrations and measuring internal concentrations of adenine nucleotides and the excretion of organic acids. The strain of H. diminuta used was found to excrete mainly lactic acid and acetic acid. Succinic acid production was generally less than 5–10% of the total. At high CO2 concentrations, the rate of excretion of lactic acid decreased while that of succinic acid increased, which conforms with the hypothesis. Acetic acid excretion did not vary significantly over the range of CO2 concentrations used. Other results did not support the hypothesis. High CO2 levels reduced the total amounts of acids excreted and the rate of succinic acid excretion was so small as to be ineffective in preventing the accumulation of H+ ions. When present in the incubation medium, succinic acid was taken up by H. diminuta. Lactic and acetic acid excretion was always sufficient to limit the accumulation of H+ ions. The conditions of incubation were shown not to be responsible for the low rates of succinic acid excreted. Incubation conditions and metabolic end-products were found to affect the rates of excretion of organic acids. There is thus a need, in work of this nature, to regulate and specify experimental conditions and to stipulate the strain of parasite used. The hypothesis was rejected and it was suggested that the energy metabolism of parasitic helminths is adapted to fluctuating O2 and CO2 tensions. 相似文献
92.
It possesses sigmoid kinetics with PEP; FBP activation changes the relationship to a rectangular hyperbola. The enzyme is inhibited by malate, which competes with PEP; FBP relieves the inhibition slightly. ATP and bicarbonate ions are also inhibitory at high concentrations. ATP inhibition is mixed-competitive with PEP; bicarbonate inhibition is non-competitive. It is suggested that pyruvate kinase may regulate both lactate and acetate production by moderating the size of the cytosolic pyruvate pool. 相似文献
93.
We have examined the effect of addition of hydroxocobalamin to growth medium on the activity of the adenosylcobalamin-requiring enzyme methylmalonyl CoA mutase in normal human fibroblasts and in mutant human fibroblasts derived from patients with inherited methylmalonicacidemia. The mutant cell lines were assigned to four distinct genetic complementation groups (cbl A, cbl B, cbl C, and cbl D), each deficient in some step in the synthesis of adenosylcobalamin from hydroxocobalamin. After control cells were grown in cobalamin-supplemented medium, mutase holoenzyme activity increased markedly in a time- and concentration-dependent fashion. Growth in cobalamin-supplemented medium had no effect on mutase activity in some mutant lines belonging to the cbl B group, while activity increased severalfold in other cbl B mutants and in all cbl A, cbl C, and cbl D mutants examined, although mutase activity was still <10% of control. Comparison of mutase holoenzyme activity and total propionate pathway activity suggests that enhancement of mutase activity in mutant cells after cobalamin supplementation to values 5–10% of control may be sufficient to overcome the inherited metabolic block and to restore total pathway activity to normal.This work was supported in part by a research grant from the National Institutes of Health (AM 12579). H. F. W. is a recipient of a traineeship from the National Institutes of Health (T01-GM02299). 相似文献
94.
95.
Studies of regulatory metabolism in Moniezia expansa: the role of phosphoenolpyruvate carboxykinase.
Phosphoenolpyruvate carboxykinase (PEPCK) from M. expansa has been partially purified and its behaviour in a range of different assay conditions has been determined. Different PEPCK's were found in the cytosol and mitochondria. Some kinetic parameters for each are presented. Both enzymes are activated by Mn2+; cytosolic PEPCK is also activated by Mg2+. The enzymes have pH optima in the range 6·4–7·0. They do not differ with respect to their apparent affinities for inosine and guanosine diphosphates, but the latter allows higher maximal activity. Little activity is observed with adenosine diphosphate. Adenosine and inosine triphosphates exert weak inhibitory effects on the Mn2+ activated enzymes; a much strongsr inhibition is exerted on the cytosolic enzyme when activated by Mg2+. A number of non-nucleotide compounds were tested for possible inhibitory effects with no success. The forward and back reactions catalyzed by PEPCK proceed at similar rates, suggesting that the enzyme may be readily raversible in vivo. 相似文献
96.
The chemotherapeutic potential of salicylhydroxamic acid (SHAM) was studied in adult rats infected with a strain of Trypanosoma brucei that kills the rats in about 100 hr. The median lethal dose, administered intraperitoneally in a carboxymethyl-cellulose suspension, is approximately 820 mg/kg body weight for male and 850 mg/kg for female rats. The apparent cause of death is severe depression of the central nervous system.Half-maximal inhibition of O2 uptake by trypanosomes in vitro requires 15 μM SHAM, whereas 100 μM inhibits over 90%. This inhibitory effect on trypanosome respiration was used as a biological assay for the effective SHAM concentration in rat plasma. After administration of a sublethal SHAM dose to rats, the effective plasma SHAM concentration rose rapidly to about 500 μM and then fell to about 10 μM at 4 hr. Nevertheless, this dose did not significantly affect the survival time of rats infected with T. brucei. Even if, by repeated SHAM administration, the plasma SHAM concentration was kept at around 100 μM for more than 4 hr, no therapeutic effect was observed.These results show that O2 uptake is not essential for the survival of trypanosomes in rats and they support the idea that bloodstream trypanosomes have an alternative pathway for glycolysis, allowing energy production in the absence of respiration.The possibility that SHAM or other inhibitors of trypanosome respiration could stilll be trypanocidal if used in conjunction with another inhibitor of glycolysis is discussed. 相似文献
97.
Effects of feeding and starvation on growth and swimming activity in an obligatory air-breathing fish 总被引:4,自引:4,他引:0
Reared in (tubular) aquaria containing different depths of water, Ophiocephalus striatus (0.7 g, 4.5 cm body length), an obligatory air-breathing tropical fish, swam long or short distances to enable themselves to exchange atmospheric air. In each tested depth (2.5, 5.0, 15.5, 31.0 and 40.0 cm) series, one group was starved, while the other was fed ad libitum twice a day on fish muscle. In the shallowest water (2.5 cm depth), the feeding group surfaced 1,294 times, travelling 64.7 m at an energy cost of 20.4 mg dry fish substance/g live fish/day, against those exposed to the deepest water (40 cm depth), which expended 35.8 mg/g/day, swimming 1,503.4 m on 1,879 visits to the surface. The starving group surfaced only 482 times, travelling 24.1 m at an expense of 5.8 mg/g/day in the shallowest water, while those at 40 cm depth surfaced 504 times, swimming 403.2 m at an energy cost of 7.4 mg/g/day. Owing to the sustained swimming activity and the consequent fatigue, the test individuals belonging to both groups in all the tested series hang to the surface for a definite interval, repaying the O2 debt. Observations were also made to assess the duration of hanging to precisely estimate the distance travelled. Irrespective of changes in depths of water, the duration of hanging to surface was only 3.0 hr/day for the feeding groups, while it was as much as 15.5 hr/day for the starving groups. The maximum sustained metabolic level of O.striatus reared in 40 cm depth was equivalent to 1.23 ml O2/g/hr, which is about 2 times higher than the value reported for the active metabolism of swimming Oncorhynchus nerka at 15°C in Brett's (1964) respirometer. O.striatus reared in 2.5 cm depth fed 32.0 mg and converted 6.7 mg dry food/g live fish/day, while those exposed to the deepest water fed 49.1 mg, but converted only 5.5 mg/g/day. Culturing obligatory air-breathing fishes in shallow waters will be advantageous. 相似文献
98.
SYNOPSIS. Euglena gracilis strain Z, green, dark-grown, and “bleached”with N-methyl-N-nitro-N-nitrosoguanidine, was found to contain 2 soluble enzymes which reduce nitrofurans. A small amount of activity was demonstrated also in a particulate fraction of sonic extracts, but none in isolated chloroplasts. The reduction of 5 nitrofurans, having widely differing bleaching activities, by each of the 2 enzymes was examined. 相似文献
99.
Evidence is presented indicating that intact plants of Hordeum vulgare degrade [α- 14C]hordenine to 14CO 2. 相似文献
100.