"Pyruvate recycling" and its influence on the estimation of the pentose pathway in intact liver and Morris hepatoma 5123TC cells

Carbohydrate metabolism in HT29 colon cancer cells cultured in a glucose free medium supplemented with inosine

1. Carbohydrate metabolism was studied in HT29 human colon cancer cells cultured in a glucose free medium supplemented with 2.8 mM inosine (HT29ino cells) in comparison with standard HT29 cells grown in the permanent presence of glucose (HT29Glc + cells) and with HT29Glc- cells which are adapted to grow permanently without glucose. 2. Inosine allows the standard cells to grow when glucose is lacking but surprisingly stops the growth of HT29Glc- cells. 3-mercaptopicolinate, an inhibitor of PEP-carboxykinase, does not hinder HT29ino cells to grow, which shows that gluconeogenesis from aspartate or pyruvate is not essential. It suggests that enough carbohydrate is supplied by the ribose moiety of inosine. 3. While standard HT29Glc + cells are highly glycolytic, it is not the case of HT29ino or HT29Glc- cells when glucose is given for few hours. When glucose is present for 24 hr or more, glycolytic rate increases in HT29ino cells and glycogen accumulates. 4. It is found that the pattern of enzymes activities related to carbohydrate metabolism in HT29ino cells is closer to that of HT29Glc + cells rather than to that of HT29Glc- cells. However, phosphofructokinase-1 activity, measured with saturating concentration of Fru-2,6-diP, is significantly lower in HT29ino cells. 5. Binding rate of hexokinase to mitochondria is similar in the three cell-lines. However, in HT29Glc- cells, bound hexokinase easily utilizes ATP generated by the mitochondria. By contrast, in HT29Glc+ and HT29ino cells, bound hexokinase is much more active with exogenous ATP, suggesting a functional defect in the mitochondria from these two latter cells.

Introduction and metabolism of pentose and hexose phosphates in permeabilized Morris hepatoma 5123TC cells

Metabolism of arabinose 5-P, ribose 5-P and glucose 6-P in permeabilized and resealed Morris hepatoma 5123TC cells was investigated by measuring the contribution of these compounds to nucleic acid biosynthesis. The level of [14C]-arabinose (non-phosphorylated) incorporation into nucleic acids was slight, presumably due to the low activity of the transport system or the absence or low activity of a specific 'kinase' enzyme. The permeabilizing procedure involved the brief treatment of Morris hepatoma 5123TC cells with lysolecithin and resulted in a cell population which was permeable to charged compounds i.e. sugar phosphates and nucleotides, that otherwise could not cross the plasma membrane. The permeabilized (and resealed cells) retained normal cellular morphology and intactness of specific organelles as judged by the maintenance of functional properties. Following permeabilization, these cells resealed when transferred back to normal growth medium, and continued to divide and increase at the same rates as control non-permeabilized cell cultures. The permeabilized cells incorporated deoxyribonucleotides ([methyl -3H]-TTP) into DNA at a linear rate of 0.047 nmol per 10(7) cells min-1, representing 90-100 per cent of the DNA synthesis rate in vivo. The permeabilization technique, when coupled with procedures to establish cell synchrony, permitted the comparative estimate of the contributions of [14C]-labelled arabinose 5-P, ribose 5-P and glucose 6-P to RNA, DNA, amino acids, CO2, lactate and sugar mono- and bisphosphates. The percentage of [14C]-isotope incorporated into total nucleic acids by these three labelled sugar phosphates were 2.3, 4.9 and 6.3 respectively. Possible reasons for the lower incorporation of 14C from arabinose 5-P are given. The results are consistent with the proposal that arabinose 5-P, an intermediate of the L-type pentose pathway activity of 5123TC cells, was incorporated into nucleic acids by its interconversion with ribulose 5-P and ribose 5-P and thus into PRPP. This study represents the first report of sugar phosphate as opposed to free sugar metabolism by tumour cells in culture.

Mechanism and quantitative contribution of the pentose pathway to the glucose metabolism of Morris hepatoma 5123C

An investigation of the mechanism and quantitative contribution of the pentose phosphate pathway in the glucose metabolism of Morris Hepatoma 5123C is reported. Morris Hepatoma 5123C has an active non-oxidative segment of pentose pathway as judged by its ability to convert ribose 5-P to hexose 6-P in a standard assay. Based on compliance with qualitative and quantitative criteria, the cells exhibit the L-type pentose pathway reaction sequence rather than the F-type pathway. This compliance included the formation of intermediates characteristic of the L-type pathway, namely arabinose 5-P, octulose mono- and bisphosphates and sedoheptulose 1,7-bisphosphate, during the dissimilation of ribose 5-P to hexose 6-P. The intermediary role of arabinose 5-P was suggested by the incorporation of its carbon into various intermediates and products of the pentose pathway. Intermediary roles for ido octulose mono- and bisphosphates were supported by their participation in the reaction catalyzed by the phosphotransferase enzyme of the L-type pentose pathway. Presence of L-type PP reactions was further affirmed by 14C-prediction labelling experiments using [5-14C]- and [2-14C]glucose as specifically labelled substrates. Using two methods of measurement, the F-type pentose cycle made a negligibly small contribution to glucose metabolism, while the measured value of the L-type pentose pathway accounted for 30% (approx.) of the total glucose metabolism of these cells, a value consistent with the high activity of the enzymes of the L-type pentose pathway in Morris Hepatoma 5123C cells and the very high activity of the non-oxidative segment of the pathway in vitro. The findings validate the proposal that the L-type pentose pathway reactions constitute the non-oxidative segment of the pathway in Morris Hepatoma 5123C cells. Reasons involving pyruvate recycling reactions show why there is low incorporation of 14C-isotope in C-1 of glucose 6-P, when [4,5,6-14C]glucose and [6-14C]glucose are L-type PP test substrates in intact cells.