Glutamate, glutamine, aspartate, asparagine, glucose and ketone-body metabolism in chick intestinal brush-border cells

Abstract

1. Suspensions of isolated chick jejunal columnar absorptive (brush-border) cells respired on endogenous substrates at a rate 40% higher than that shown by rat brush-border cells. 2. Added d-glucose (5 or 10mm), l-glutamine (2.5mm) and l-glutamate (2.5mm) were the only individual substrates which stimulated respiration by chick cells; l-aspartate (2.5 or 6.7mm), glutamate (6.7mm), glutamine (6.7mm), l-alanine (1 or 10mm), pyruvate (1 or 2mm), l-lactate (5 or 10mm), butyrate (10mm) and oleate (1mm) did not stimulate chick cell respiration; l-asparagine (6.7mm) inhibited slightly; glucose (5mm) stimulated more than did 10mm-glucose. 3. Acetoacetate (10mm) and d-3-hydroxybutyrate (10mm) were rapidly consumed but, in contrast to rat brush-border cells, did not stimulate respiration. 4. Glucose (10mm) was consumed more slowly than 5mm-glucose; the dominant product of glucose metabolism during vigorous respiration was lactate; the proportion of glucose converted to lactate was greater with 10mm- than with 5mm-glucose. 5. Glutamate and aspartate consumption rates decreased, and alanine and glutamine consumption rates increased when their initial concentrations were raised from 2.5 to 6.7 or 10mm. 6. The metabolic fate of glucose was little affected by concomitant metabolism of any one of aspartate, glutamate or glutamine except for an increased production of alanine; the glucose-stimulated respiration rate was unaffected by concomitant metabolism of these individual amino acids. 7. Chick cells produced very little alanine from aspartate and, in contrast to rat cells, likewise produced very little alanine from glutamate or glutamine; in chick cells alanine appeared to be predominantly a product of transmination of pyruvate derived from glucose metabolism. 8. In chick cells, glutamate and glutamine were formed from aspartate (2.5 or 6.7mm); aspartate and glutamine were formed from glutamate (2.5mm) but only aspartate from 6.7mm-glutamate; glutamate was the dominant product formed from glutamine (6.7mm) but aspartate only was formed from 2.5mm-glutamine. 9. Chick brush-border cells can thus both catabolize and synthesize glutamine; glutamine synthesis is always diminished by concomitant metabolism of glucose, presumably by allosteric inhibition of glutamine synthetase by alanine. 10. Proline was formed from glutamine (2.5mm) but not from glutamine (2.5mm)+glucose (5mm) and not from 2.5mm-glutamate; ornithine was formed from glutamine (2.5mm)+glucose (5.0mm) but not from glutamine alone; serine was formed from glutamine (2.5mm)+glucose (5mm) and from these two substrates plus aspartate (2.5mm). 11. Total intracellular adenine nucleotides (22μmol/g dry wt.) remained unchanged during incubation of chick cells with glucose. 12. Intracellular glutathione (0.7–0.8mm) was depleted by 40% during incubation of respiring chick cells without added substrates for 75min at 37°C; partial restoration of the lost glutathione was achieved by incubating cells with l-glutamate+l-cysteine+glycine.