Open AccessRecycling of Glucose by Rat Hepatocytes
Author(s) -
KATZ Joseph,
WALS P. A.,
GOLDEN Sybil,
ROGNSTAD Robert
Publication year - 1975
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1111/j.1432-1033.1975.tb20979.x
Subject(s) - glycogen , tritiated water , chemistry , glucose uptake , tritium , yield (engineering) , substrate (aquarium) , carbohydrate metabolism , metabolism , dihydroxyacetone , medicine , d glucose , gluconeogenesis , biochemistry , dihydroxyacetone phosphate , endocrinology , biology , enzyme , insulin , glycerol , ecology , physics , materials science , nuclear physics , metallurgy
1 The metabolism of glucose labeled uniformly with 14 C, and in positions 2, 3 and 5 with tritium by hepatocytes from fed and fasted rats were studied. Cells were incubated with glucose as sole substrate, or with glucose and a variety of glucose precursors, and uptake or production of glucose, and the utilization of the isotopes was determined. 2 There was no uptake of glucose at concentration of up to 15 mM, and net glucose synthesis in the presence of precursors. 14 C was however recovered in CO 2 , lactate and amino acids, and tritium in water. Considerable incorporation into glycogen from 14 C and 3 H‐labeled glucose occurred at high (above 20 mM) glucose concentrations. 3 The yield in water always exceeded that in 14 C‐labeled products. The yield in 3 HOH from [2‐ 3 H]glucose exceeded that from [5‐ 3 H]glucose, and the latter was greater than from [3‐ 3 H]glucose. 4 Utilization of labeled glucose does not follow Michaelis‐Menten kinetics. The fractional rate of uptake of 14 C and tritium‐labeled glucose increases with glucose concentration with a maximum at about 15 mM and then declines. 5 The effect of numerous gluconeogenic substrates on the isotope utilization and the 3 H/ 14 C ratio in glycogen was studied. The uptake of 14 C was always depressed. Addition of lactate and dihydroxyacetone has little effect on the detritiation of [2‐ 3 H]glucose, but it is depressed by other substrates. The detritiation of [3‐ 3 H]‐ and [5‐ 3 H]glucose is depressed in aluconeogenesis, that from [3‐ 3 H]glucose usually more than from [5‐ 3 H]glucose. In the presence of lactate detritiation of [3‐ 3 H]glucose is about half that from [5‐ 3 H]glucose. 6 Equations to calculate the phosphorylation of glucose and fructose 6‐phosphate in the presence of futile cycling between glucose and glucose 6‐phosphate and fructose 6‐phosphate and fructose 1,6‐bisphosphate were derived. 7 The estimate of glucose phosphorylation requires determination of the specific activity of glucose 6‐phosphate from [2‐ 3 H]glucose. It appears that. futile cycling between glucose and glucose 6‐phosphate is extensive in cells with a high glycogen content, but is low in cells from starved rats and nearly absent in those from diabetic animals. 8 The estimation of the phosphorylation of fructose 6‐phosphate in the presence of cycling requires knowledge of the specific activities of fructose 6‐phosphate and fructose 1,6‐bisphosphate from [3‐ 3 H]glucose. At present there are no adequate data to calculate phosphorylation and recycling of fructose 6‐phosphate, but under some conditions the rate may be quite high.