Conditions Of Limited Oxygen Availability Research Articles

Shrunken-1-encoded sucrose synthase is not required for the sucrose-ethanol transition in maize under anaerobic conditions

The physiological role of the strong anaerobic induction of the sucrose synthase gene, Shrunken-1 (Sh1), is still unknown. It is likely that sucrose synthase plays an important role in the cytosolic utilization of sucrose to ethanol under conditions of limited oxygen availability, via the fermentative pathway. In this paper we examined the importance of the Sh1-encoded sucrose synthase for the utilization of sucrose through the alcoholic fermentative pathway in maize. The results indicate that deficiency in the Sh1-encoded sucrose synthase does not have any effect on the rate of ethanol production. Unexpectedly, this is not the result of enhanced invertase activity in the sh1 mutant but, instead, of the presence of Sus1-encoded sucrose synthase, which is normally absent in wild-type maize seedlings.

Surface Effects on Deposits From Jet Fuels

Flow experiments in a single-pass heat exchanger using JP-8 and certain additives were initiated under controlled conditions to explore the effects of a metal surface on deposition. The experimental apparatus permitted a unique viewing of the time evolution of deposits at different axial locations under conditions of limited oxygen availability somewhat similar to those in jet aircraft. Scanning electron microscopy was used to examine deposit microstructure. In addition, x-ray photoelectron spectroscopy and Auger electron spectroscopy determined the chemical composition of the deposits. Oxygen concentration measurements in the bulk flow were also performed, and the observed transient oxidation behavior was related to measured time-dependent changes in the deposit. Increasing dissolved oxygen levels and large changes in deposition were characteristic of the induction time. Mechanisms of fouling in the heated and cooled sections were different. Spectroscopic analysis indicated that deposits formed in the heated section had chemical compositions different from those formed in the cooled section. Scanning electron microscopy revealed differences in microstructure between the heated and cooled sections: More uniform deposits formed in the cooled section as a result of once-soluble species becoming insoluble at low temperatures. In addition, the JP-8 additives significantly reduced fouling in the heated section, but their effectiveness in the cooled section, especially after long periods, was unclear.

Amylolytic Activities in Cereal Seeds under Aerobic and Anaerobic Conditions.

An adequate carbohydrate supply contributes to the survival of seeds under conditions of limited oxygen availability. The amount of soluble, readily fermentable carbohydrates in dry cereal seeds is usually very limited, with starch representing the main storage compound. Starch breakdown during the germination of cereal seeds is the result of the action of hydrolytic enzymes and only through the concerted action of [alpha]-amylase (EC 3.2.1.1), [beta]-amylase (EC 3.2.1.2), debranching enzyme (EC 3.2.1.41), and [alpha]-glucosidase (EC 3.2.1.20) can starch be hydrolyzed completely. We present here data concerning the complete set of starch-degrading enzymes in three cereals, rice (Oryza sativa L.), which is tolerant to anaerobiosis, and wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.), which are unable to germinate under anoxia. Among the cereal seeds tested under anoxia, only rice is able to degrade nonboiled, soluble starch, reflecting the ability to degrade the starch granules in vivo. This is explained by the presence of the complete set of enzymes needed to degrade starch completely either as the result of de novo synthesis ([alpha]-amylase, [beta]-amylase) or activation of preexisting, inactive forms of the enzyme (debranching enzyme, [alpha]-glucosidase). These enzymes are either absent or inactive in wheat and barley seeds kept under anaerobic conditions.

Sickle cell trait performance in a prolonged race at high altitude

The limitation of aerobic exercise capacity of athletes with the sickle cell trait (SCT), under conditions of limited oxygen availability, is still controversial. To study this, we took advantage of an unique setting, the International Mount Cameroon Ascent Race, a 34.1-km race over difficult terrain, slopes ranging from 7 to 40%, and altitudes varying from 615 to 4095 m, combined with high prevalence rates of SCT among the African runners. Of 266 Cameroonian runners, SCT was detected in 33 athletes (12.4%), a prevalence similar to that of the ethnically corrected general population. However, in runners of the Bakoueri tribe whose performance is contingent with social stature, SCT was present in only 1 of 41 runners (2.4%), as compared with 15.6% in the general population of the Bakoueri tribe (P < 0.03). In general, performance times of SCT runners were not different from non-SCT runners, except during the portion of the race at altitudes ranging from 3800 to 4095 m, where significantly longer times were clocked by SCT subjects (P < 0.02). We conclude that prolonged aerobic efforts in hypobaric hypoxic conditions may be associated with a detrimental effect on performance in SCT carriers. If this is true, it might account for the reduced prevalence of SCT among those runners representing the Bakoueri tribe, provided an objective measure of performance at altitude was employed to select these representatives.

Ignition Propensity Testing at Different O2 Concentrations

Ignition propensity of cigarettes on soft furnishings is a function of oxygen availability. Oxygen availability, in turn, depends on air movement, and substrate permeability and geometry. Experiments are described where oxygen concentrations over a range from 14 to 36% were used to control oxygen availability and thereby simulate the effects of the other above named parame ters. The experiments were carried out with two cigarettes (high and low tobacco density and paper permeability), two fabrics (Denim and California Standard, both treated with a range of levels of potassium acetate to obtain an array of different ignitabilities), and two configurations (Flat and L). Each cigarette was characterized in terms of an Ignition Oxygen Level (IOL) (minimum O2 concentration required for ignition) and an Extinction Oxygen Level (EOL) (maximum O 2 concentration below which the cigarettes self- extinguish). The IOL is a function of fabric ignitability, and the EOL is a func tion of fabric permeability. When the results are plotted on a grid of Fabric Ig nitability (FI) versus Oxygen Availability (OA) three domains result: (1) combinations of FI and OA where ignitions take place, (2) combinations of FI and OA where the cigarette self-extinguishes, and (3) combinations of FI and OA where the cigarette burns its whole length without causing an ignition. The experiments demonstrated that when cigarettes are ranked according to ignition propensity, the rankings can reverse depending on test conditions. Thus, of the two test cigarettes, the high density/paper permeability cigarette was more ignition-prone under conditions of limited oxygen availability, whereas the low density/paper permeability cigarette became more ignition- prone as oxygen availability increased.

A theoretical analysis of the effect of placental metabolism on fetal oxygenation under conditions of limited oxygen availability

The purpose of this theoretical paper is to examine the effects of placental metabolism on fetal oxygenation under conditions of limited oxygen availability. Features of the mathematical model used here include: (1) ordinary non-linear differential equations defining the oxygen partial pressure profiles in the maternal and fetal streams for a concurrent flow pattern; (2) the presence of maternal and fetal blood flow shunts; (3) consumption of oxygen by a metabolically active placenta; and (4) modification of the fetal input to the placenta by changing the rate of fetal oxygen consumption in response to changes in the rate of oxygen delivered to the fetus via the umbilical vein. Model parameters were chosen to be well within the range of values cited in the literature. Based on these calculations, we conclude that: (1) under normal conditions, approximately one-half of the fetal uterine-umbilical venous oxygen partial pressure difference can be attributed to placental oxygen consumption; (2) utilization of fetal oxygen to help maintain the metabolic activities of the placenta does not significantly impair fetal oxygenation under normal conditions; (3) consumption of oxygen by the placenta will have a significant detrimental effect on the rate of oxygen delivered to the fetus if oxygen availability is compromised; and (4) for the same rate of maternal oxygen delivered to the placenta, maternal hypoxemia has a significantly greater adverse effect on fetal oxygenation than does maternal anemia.