A KINETIC ANALYSIS OF THE ENDOGENOUS RESPIRATION OF BAKERS' YEAST

The process of endogenous respiration of two strains of bakers'' yeast, Saccharomyces cerevisiae, was examined kinetically. The rate of respiration with respect to time in a non-nutrient medium was found to exhibit two phases: (a) a period of constant rate of O₂ consumption and CO₂ production (R.Q. = 1) characteristic of cells with ample concentrations of stored material; (b) a first order decline in rate of respiration with respect to time, where the rate was proportional to the concentration of some substrate, S. (R.Q. = 1 throughout second phase.) The nature of this substrate was reexamined and the evidence summarized confirms the notion that it is a carbohydrate, probably glycogen. These phases of endogenous respiration were shown to depend upon the age of the culture and the amount of substrate available.     

不同糖源及糖水平对大菱鲆糖代谢酶活性的影响

采用34双因素实验设计, 以初始质量为(8.060.08) g的大菱鲆幼鱼(Scophthalmus maximus L.)为对象, 研究在饲料中添加3种糖源(葡萄糖、蔗糖和糊精)及4个水平(0、5%、15%、28%)对大菱鲆肝脏糖酵解关键酶己糖激酶(HK)、葡萄糖激酶(GK)、磷酸果糖激酶(PFK)、丙酮酸激酶(PK)和糖异生关键酶磷酸烯醇式丙酮酸羧激酶(PEPCK)、1, 6-二磷酸果糖酶(FBPase)活性的影响。结果表明: 饲料糖添加量从0升高到15%时, 大菱鲆的糖酵解酶GK和PK活性随饲料葡萄糖或糊精含量的增加而增加; 当饲料中葡萄糖或糊精含量为28%时, GK和PK活性有下降的趋势。3种糖源的4个添加水平对HK和PFK活性均无显著影响(P 0.05)。添加不同水平的葡萄糖对大菱鲆糖异生途径的PEPCK活性无显著影响(P 0.05), 但在饲料中葡萄糖添加量为5%时显著促进了FBPase活性(P 0.05), 当葡萄糖添加量升高为15%或28%时, FBPase活性与对照组无显著差异(P 0.05)。糊精作为饲料糖源时抑制了大菱鲆肝脏FBPase和PEPCK的活性, 而添加不同水平的蔗糖对FBPase和PEPCK活性的影响均不显著(P 0.05)。总的来说, 从大菱鲆幼鱼肝脏糖代谢角度而言, 在饲料中添加15%的葡萄糖或糊精时, 可以有效促进大菱鲆肝脏糖酵解能力; 较添加葡萄糖, 糊精在促进大菱鲆肝脏糖酵解的同时对糖异生存在一定程度的抑制。蔗糖作为饲料糖源时, 仅在添加量为28%时显著促进糖酵解酶GK活性, 糖酵解其他酶活性以及糖异生酶活性均不受蔗糖水平的显著影响。       

羊草根茎的贮藏碳水化合物及对氮素添加的响应

 为了研究氮素对羊草(Leymus chinensis)根茎碳水化合物贮藏的影响,在中国科学院内蒙古草原生态系统定位研究站的羊草样地,设计了不同水平和不同时期的氮素添加试验。采用高效液相色谱（High Performance Liquid Chromatography, HPLC）对羊草根茎中的贮藏性碳水化合物进行了测定。结果表明,羊草根茎中的贮藏碳水化合物组分包括果聚糖、甘露糖醇、蔗糖、葡萄糖和果糖。其中果聚糖是最主要贮藏碳水化合物,约占60%;其次是甘露糖醇,约占20%。氮素添加量对羊草根茎中的贮藏碳水化合物有显著影响。在0～17.5 g N·m－2范围内,随着氮素添加量的增加,碳水化合物总量、果聚糖、甘露糖醇的含量均逐渐升高。氮素添加时期对羊草根茎中的贮藏碳水化合物的含量亦有显著影响。在7月初添加氮素比4月份添加氮素更有利于贮藏碳水化合物的积累。 关键词     

METABOLIC INTERMEDIATES IN ADAPTIVE FERMENTATION OF GALACTOSE BY YEAST

Pyruvic acid, which is known to be an intermediate of glucose fermentation, was added to yeast during adaptation to galactose fermentation. It was found to neutralize the inhibition by sodium fluoride, and to decrease the apparent time of adaptation from 90 to about 45 or 60 minutes. In control experiments, it was shown that intact yeast is unable appreciably to ferment or decarboxylate alone, although it oxidizes the compound readily. Experiments in which galactose and pyruvate were added at various times and in different orders were used to eliminate the possible complications of the rates at which these compounds penetrate the cells. Under these conditions, it was not possible to reduce the time of adaptation below 45 minutes. It was concluded that the rôle of added pyruvate was to serve as a source of acetaldehyde, which in turn could accept hydrogen and be reduced to alcohol. Substances, such as triose phosphate, which could serve as hydrogen donors were not produced from galactose in appreciable quantities until 45 minutes had elapsed. This time was therefore inferred to be the true adaptation time, during which the first synthesis of adaptive enzymes occurred. Some determinations of the distribution of phosphorylated intermediates at various stages during the adaptive process were carried out. It was found that ATP, which usually serves to phosphorylate hexoses, accumulates during the preadaptive phase, diminishes rapidly after 60 minutes, and subsequently increases once more. The source of the ATP phosphate appeared to be PPA or triose phosphate initially present in the cells. It was inferred that the adaptive enzyme was concerned with the phosphorylation of galactose and the conversion of the phosphate ester to a glucose ester, which could then be fermented by the normal enzymes of the cell. Added ATP was found to stimulate adaptation to a considerable extent, but did not shorten the time of adaptation below 75 minutes. This seemed consistent with the rôle of ATP as a phosphate donor for galactose. Creatine was found to inhibit adaptation to some degree, in agreement with its known ability to act as a competitive phosphate acceptor. It was demonstrated that yeast produces, during and after adaptation, substances which shorten the apparent adaptation time of fresh samples of yeast. In agreement with our other findings, it appeared that such substances were not formed before about 45 minutes. They are probably not metabolic intermediates, and may be identical with the adaptive principle which can be extracted from adapted cells.     

SUCROSE INVERSION BY BAKERS' YEAST AS A FUNCTION OF TEMPERATURE

Inversion of sucrose by bakers'' yeast follows the same course as inversion catalyzed by yeast invertase. Rate of inversion increases exponentially with temperature; the temperature characteristic in the Arrhenius equation is 10,700 below 13–17°C., and 8,300 above that temperature. Temperature inactivation occurs above 40°C. The effects of temperature upon rate of inversion were the same using Fleischmann''s yeast cake, the same yeast killed with toluene, and a pure strain (G. M. No. 21062) of bakers'' yeast. The last differed from the other two only in the fact that its critical temperature was 13°C. as compared with 17°C. for the others. The catalytic inversion is associated with enzyme activity inside the cell, not in the medium, and is independent of any vital processes inside the cell such as respiration and fermentation. Since invertase activity is the same inside the cell as it is after extraction, it appears possible to relate the temperature characteristics for physiological processes to the catalytic chemical systems which determine their rate. At least two enzymes are capable of inverting sucrose in the yeast cell. The familiar yeast invertase (µ = 10,700) is active below 13–17°C. while a second enzyme (M = 8,300) plays the dominant role above that temperature.     

METABOLIC CONTROL MECHANISMS IN MAMMALIAN SYSTEMS

Abstract— The regulation by thyroid hormone of the activities of hexokinase (ATP: D-hexose 6-phosphotransferase; EC 2.7.1.1), phosphofructokinase (ATP: D-fructose-6- phosphate 1-phosphotransferase; EC 2.7.1.11) and pyruvate kinase (ATP: pyruvate phosphotransferase; EC 2.7.1.40) has been investigated in the soluble fractions of the cerebral cortex and cerebellum of the rat. Ontogenetic studies on these key glycolytic enzymes demonstrated marked increases in the normal cerebral cortex between 1 day and 1 yr of age; less pronounced increases in enzyme activities were noted in the normal cerebellum. Neonatal thyroidectomy, induced by treatment of 1-day-old rats with 100 μCi of ¹³¹I, ied to an impairment of body and brain growth and inhibited the developmental increases in hexokinase, phosphofructokinase and pyruvate kinase in both the cerebral cortex and cerebellum. Whereas 50 μCi of ¹³¹I had little or no effect on these brain enzymes, 200 μCi of the radioisotope markedly inhibited (35–65 per cent) the developmental increases of the various enzyme activities investigated. When administration of the radioisotope was delayed for 20 days after birth, little or no inhibition of the development of brain glycolytic enzymes was observed. Whereas treatment of normal neonatal animals with L-tri-iodothyronine had no significant effect on the activities of cerebro-cortical and cerebellar glycolytic enzymes, the hormone increased their activities in young cretinous rats. However, when the initiation of tri-iodothyronine treatment was delayed until neonatally thyroidectomized rats had reached adulthood, this hormone failed to produce any appreciable change in enzyme activity. Our results indicate that thyroid hormone exerts an important regulatory influence on the activities of hexokinase, phosphofructokinase and pyruvate kinase in the developing cerebral cortex and cerebellum.     

THE COST OF SEXUAL SIGNALING IN YEAST

The handicap principle holds that costly sexual signals can reliably indicate mate quality. Only individuals of high quality can afford a strong signal—the cost of signaling is relatively lower for high‐quality signalers than for low‐quality signalers. This critical property is difficult to test experimentally because the benefit of signaling on mating success, and cost of signaling on other components of fitness, cannot easily be separated in obligate sexual organisms. We therefore studied the facultatively sexual yeast Saccharomyces cerevisiae, which produces pheromones to attract potential mates. To precisely measure the cost of signaling, the signal was reduced or removed by deleting one or both copies of the pheromone‐encoding genes and measuring asexual growth rate in competition with a wild‐type signaler. We manipulated signaler quality either by changing the quality of the assay environment or by changing the number of deleterious mutations carried. For both types of treatment, we found that the cost of signaling decreased as the quality of the signaler increased, demonstrating that the yeast pheromone signal has the key property required for selection under the handicap principle. We found that cells of high genetic quality produce stronger signals than low‐quality cells, verifying that the signal is indeed honest.