The utility of the 13C-galactose breath test as a measure of liver function.

BACKGROUND: The 13C-galactose breath test has been reported to be an accurate, non-invasive method for the assessment of liver function. AIMS: To determine the optimal doses of labelled and unlabelled carrier galactose necessary to perform the 13C-galactose breath test, to assess the utility of the 13C-galactose breath test in distinguishing between normal subjects and those with liver cirrhosis and to determine whether the 13C-galactose breath test can stratify patients with cirrhosis based on their Child-Pugh score. METHODS: Twenty-three control subjects and 30 patients with liver cirrhosis received fixed doses of unlabelled carrier galactose and labelled 13C-galactose. Breath samples were collected just before and at 30-min intervals up to 4 h after the ingestion of unlabelled carrier galactose and labelled 13C-galactose. Each sample was analysed for its 13CO2 content. RESULTS: Doses of 25 g/m2 of unlabelled carrier galactose and 100 mg of 13C-galactose had the greatest sensitivity (93%; 95% confidence interval, 76-99%) and specificity (87%; 95% confidence interval, 65-97%) for distinguishing between normal subjects and cirrhotics when the test was performed 2 h after ingestion. The 13C-galactose breath test was also able to distinguish between class A and class B or C cirrhotics. CONCLUSION: The 13C-galactose breath test is a useful non-invasive tool for distinguishing between healthy subjects and patients with liver cirrhosis and between cirrhotics with well-compensated liver disease and those with decompensated liver disease.

The effect of low osmotic potential on nitrite reduction in intact spinach chloroplasts.

The effect of water stress (reduced osmotic potential) on photosynthetic nitrite reduction was investigated using intact, isolated spinach (Spinacia oleracea) chloroplasts. Nitrite-dependent O(2) evolution was inhibited 39% at -29.5 bars osmotic potential, relative to a control at -11 bars. In the presence of an uncoupler of photophosphorylation this inhibition was not seen. Reduced osmotic potential did not inhibit either methyl viologen reduction or photosynthetic O(2) reduction. These results indicate that an inhibition of electron transport to ferredoxin cannot account for the observed inhibition of nitrite-dependent O(2) evolution. In vitro assay of nitrite reductase activity showed that the interaction of the enzyme with nitrite was not affected by changes in the concentrations of ions or molecules that might be caused by water stress conditions. These results indicate that the most likely site for the effect of water stress on chloroplastic nitrite reduction is the interaction of ferredoxin with nitrite reductase.

Photosynthetic o(2) exchange kinetics in isolated soybean cells.

Light-dependent O(2) exchange was measured in intact, isolated soybean (Glycine max. var. Williams) cells using isotopically labeled O(2) and a mass spectrometer. The dependence of O(2) exchange on O(2) and CO(2) was investigated at high light in coupled and uncoupled cells. With coupled cells at high O(2), O(2) evolution followed similar kinetics at high and low CO(2). Steady-state rates of O(2) uptake were insignificant at high CO(2), but progressively increased with decreasing CO(2). At low CO(2), steady-state rates of O(2) uptake were 50% to 70% of the maximum CO(2)-supported rates of O(2) evolution. These high rates of O(2) uptake exceeded the maximum rate of O(2) reduction determined in uncoupled cells, suggesting the occurrence of another light-induced O(2)-uptake process (i.e. photorespiration).Rates of O(2) exchange in uncoupled cells were half-saturated at 7% to 8% O(2). Initial rates (during induction) of O(2) exchange in uninhibited cells were also half-saturated at 7% to 8% O(2). In contrast, steady-state rates of O(2) evolution and O(2) uptake (at low CO(2)) were half-saturated at 18% to 20% O(2). O(2) uptake was significantly suppressed in the presence of nitrate, suggesting that nitrate and/or nitrite can compete with O(2) for photoreductant.These results suggest that two mechanisms (O(2) reduction and photorespiration) are responsible for the light-dependent O(2) uptake observed in uninhibited cells under CO(2)-limiting conditions. The relative contribution of each process to the rate of O(2) uptake appears to be dependent on the O(2) level. At high O(2) concentrations (>/=40%), photorespiration is the major O(2)-consuming process. At lower (ambient) O(2) concentrations (

Photosynthetic oxygen reduction in isolated intact chloroplasts and cells in spinach.

The time course of light-induced O(2) exchange by isolated intact chloroplasts and cells from spinach was determined under various conditions using isotopically labeled O(2) and a mass spectrometer. In dark-adapted chloroplasts and cells supplemented with saturating amounts of bicarbonate, O(2) evolution began immediately upon illumination. However, this initial rate of O(2) evolution was counterbalanced by a simultaneous increase in the rate of O(2) uptake, so that little net O(2) was evolved or consumed during the first approximately 1 minute of illumination. After this induction (lag) phase, the rate of O(2) evolution increased 3- to 4-fold while the rate of O(2) uptake diminished to a very low level. Inhibition of the Calvin cycle, e.g. with dl-glyceraldehyde or iodoacetamide, had negligible effects on the initial rate of O(2) evolution or O(2) uptake; both rates were sutained for several minutes, and about balanced so that no net O(2) was produced. Uncouplers had an effect similar to that observed with Calvin cycle inhibitors, except that rates of O(2) evolution and photoreduction were stimulated 40 to 50%.These results suggest that higher plant phostosynthetic preparations which retain the ability to reduce CO(2) also have a significant capacity to photoreduce O(2). With near-saturating light and sufficient CO(2), O(2) reduction appears to take place primarily via a direct interaction between O(2) and reduced electron transport carriers, and occurs principally when CO(2)-fixation reactions are suboptimal, e.g. during induction or in the presence of Calvin cycle inhibitors. The inherent maximum endogenous rate of O(2) reduction is approximately 25 to 50% of the maximum rate of noncyclic electron transport coupled to CO(2) fixation. Although the photoreduction of O(2) is coupled to ion transport and/or phosphorylation, this process does not appear to supply significant amounts of ATP directly during steady-state CO(2) fixation in strong light.