Changes In Maximal Activities Research Articles

Effects of Elevated CO2 on Levels of Primary Metabolites and Transcripts of Genes Encoding Respiratory Enzymes and Their Diurnal Patterns in Arabidopsis thaliana: Possible Relationships with Respiratory Rates

Elevated CO2 affects plant growth and photosynthesis, which results in changes in plant respiration. However, the mechanisms underlying the responses of plant respiration to elevated CO2 are poorly understood. In this study, we measured diurnal changes in the transcript levels of genes encoding respiratory enzymes, the maximal activities of the enzymes and primary metabolite levels in shoots of Arabidopsis thaliana grown under moderate or elevated CO2 conditions (390 or 780 parts per million by volume CO2, respectively). We examined the relationships between these changes and respiratory rates. Under elevated CO2, the transcript levels of several genes encoding respiratory enzymes increased at the end of the light period, but these increases did not result in changes in the maximal activities of the corresponding enzymes. The levels of some primary metabolites such as starch and sugar phosphates increased under elevated CO2, particularly at the end of the light period. The O2 uptake rate at the end of the dark period was higher under elevated CO2 than under moderate CO2, but higher under moderate CO2 than under elevated CO2 at the end of the light period. These results indicate that the changes in O2 uptake rates are not directly related to changes in maximal enzyme activities and primary metabolite levels. Instead, elevated CO2 may affect anabolic processes that consume respiratory ATP, thereby affecting O2 uptake rates.

A Distributed Model of Carbohydrate Transport and Metabolism in the Liver during Rest and High-Intensity Exercise

A model of reaction and transport in the liver was developed that describes the metabolite concentration and reaction flux dynamics separately within the tissue and blood domains. The blood domain contains equations for convection, axial dispersion, and transport to the surrounding tissue; and the tissue domain consists of reactions representing key carbohydrate metabolic pathways. The model includes the metabolic heterogeneity of the liver by incorporating spatial variation of key enzymatic maximal activities. Simulation results of the overnight fasted, resting state agree closely with experimental values of overall glucose uptake and lactate output by the liver. The incorporation of zonation of glycolytic and gluconeogenic enzyme activities causes the expected increase in glycolysis and decrease in gluconeogenesis along the sinusoid length from periportal to perivenous regions, while fluxes are nearly constant along the sinusoid length in the absence of enzyme zonation. These results confirm that transport limitations are not sufficient to account for the observed tissue heterogeneity of metabolic fluxes. Model results indicate that changes in arterial substrate concentrations and hepatic blood flow rate, which occur in the high-intensity exercise state, are not sufficient to shift the liver metabolism enough to account for the 5-fold increase in hepatic glucose production measured during exercise. Changes in maximal activities, whether caused by exercise-induced changes in insulin, glucagon, or other hormones are shown to be needed to achieve the expected glucose output. This model provides a framework for evaluating the relative importance to hepatic function of various phenomenological changes that occur during exercise. The model can also be used to assess the potential effect of metabolic heterogeneity on metabolism.

Importance of Substrate Changes in the Decrease of Hepatic Glucose Cycling During Insulin Infusion and Declining Glycemia in the Depancreatized Dog

We wished to determine whether the elevated glucose cycling (GC) between glucose and glucose-6-phosphate (G⇆G6P) in diabetes can be reversed with acute insulin treatment. In six insulin-deprived, anesthetized, depancreatized dogs, insulin was infused for 6–9 h at a starting dose of 45–150 pmol-kg−1-min−1 to normalize plasma glucose from 23.9 ± 1.4 to 5.0 ± 0.4 mmol/1 and gradually decreased to and maintained at a basal rate (1.7 ± 1.0 pmol · kg−1 · min−1) during the last 3 h. GC, measured with [2-3H]- and [6-3H]glucose, fell markedly from 15.3 ± 2.7 and normalized at 1.3 ± 0.6 μmol · kg−1 · min−1 (P < 0.001). This occurred because total hepatic glucose output fell much more (from 41.2 ± 3.1 to 11.6 ± 1.2) than did glucose production (from 25.9 ± 1.9 to 10.3 ± 1.0 μmol · kg−1 · min−1) (both P < 0.01). Freeze-clamped liver biopsies were taken at timed intervals for measurements of hepatic enzymes and substrates. The elevated hepatic hexose-6-phosphate levels decreased with insulin infusion (151 ± 24 vs. 71 ± 13 nmol/g, P < 0.01). Maximal activities of glucose-6-phosphatase (G6Pase) (from 17.6 ± 0.8 to 19.6 ± 2.6 U/g) and glucokinase (from 1.1 ± 0.2 to 1.0 ± 0.2 U/g) did not change. Insulin infusion resulted in a threefold increase (P < 0.05) in the activity of glycogen synthase (active form), but had no effect on hepatic glycogen content. We therefore conclude that in the depancreatized dog, 1) acute insulin infusion with a concurrent correction of hyperglycemia can markedly reduce and normalize GC, and 2) the acute reduction in GC is primarily mediated by reduced substrate fluxes through glucokinase and G6Pase and does not require stable changes in maximal activities of the two enzymes. However, in vivo allosteric changes in enzyme activities (particularly G6Pase), immeasurable in vitro, cannot be excluded. In addition to enzyme measurements, the importance of tracer methods that allow for in vivo measurements of fluxes through glucokinase and G6Pase is emphasized.

Regulation by insulin of glucose metabolism in mammary gland of anaesthetized lactating rats. Stimulation of phosphofructokinase-1 by fructose 2,6-bisphosphate and activation of acetyl-CoA carboxylase.

The effect of insulin on glucose metabolism in mammary gland was studied by the euglycaemic/hyperinsulinaemic-clamp technique. Measurement of metabolite concentrations and enzyme activities in the mammary gland suggests two sites of action of insulin: phosphofructokinase-1 and acetyl-coA carboxylase. The increase in phosphofructokinase-1 activity could be linked to the 2-fold increase in fructose 2,6-bisphosphate concentration, since no change in maximal activity and in sensitivity of the enzyme toward fructose 6-phosphate was detected in vitro.

The metabolism of L-phenylalanine and L-tyrosine by liver cells isolated from adrenalectomized rats and from streptozotocin-diabetic rats.

Flux through, and maximal activities of, key enzymes of phenylalanine and tyrosine degradation were measured in liver cells prepared from adrenalectomized rats and from streptozotocin-diabetic rats. Adrenalectomy decreased the phenylalanine hydroxylase flux/activity ratio; this was restored by steroid treatment in vivo. Changes in the phosphorylation state of the hydroxylase may mediate these effects; there was no significant change in the maximal activity of the hydroxylase. Tyrosine metabolism was enhanced by adrenalectomy; this was not related to any change in maximal activity of the aminotransferase. Steroid treatment increased the maximal activity of the aminotransferase. Both acute (3 days) and chronic (10 days) diabetes were associated with increased metabolism of phenylalanine; insulin treatment in vivo did not reverse these changes. Although elevated hydroxylase protein concentration was a major factor, changes in the enzyme phosphorylation state may contribute to differences in phenylalanine degradation in the acute and chronic diabetic states. Tyrosine metabolism, increased by diabetes, was partially restored to normal by insulin treatment in vivo. These changes can, to a large extent, be interpreted in terms of changes in the maximal activity of the aminotransferase.