Glucose 6-phosphate formation by L-type pentose phosphate pathway reactions of rat liver in vitro: further evidence

The return of metabolism: biochemistry and physiology of the pentose phosphate pathway

The pentose phosphate pathway (PPP) is a fundamental component of cellular metabolism. The PPP is important to maintain carbon homoeostasis, to provide precursors for nucleotide and amino acid biosynthesis, to provide reducing molecules for anabolism, and to defeat oxidative stress. The PPP shares reactions with the Entner–Doudoroff pathway and Calvin cycle and divides into an oxidative and non-oxidative branch. The oxidative branch is highly active in most eukaryotes and converts glucose 6-phosphate into carbon dioxide, ribulose 5-phosphate and NADPH. The latter function is critical to maintain redox balance under stress situations, when cells proliferate rapidly, in ageing, and for the ‘Warburg effect’ of cancer cells. The non-oxidative branch instead is virtually ubiquitous, and metabolizes the glycolytic intermediates fructose 6-phosphate and glyceraldehyde 3-phosphate as well as sedoheptulose sugars, yielding ribose 5-phosphate for the synthesis of nucleic acids and sugar phosphate precursors for the synthesis of amino acids. Whereas the oxidative PPP is considered unidirectional, the non-oxidative branch can supply glycolysis with intermediates derived from ribose 5-phosphate and vice versa, depending on the biochemical demand. These functions require dynamic regulation of the PPP pathway that is achieved through hierarchical interactions between transcriptome, proteome and metabolome. Consequently, the biochemistry and regulation of this pathway, while still unresolved in many cases, are archetypal for the dynamics of the metabolic network of the cell. In this comprehensive article we review seminal work that led to the discovery and description of the pathway that date back now for 80 years, and address recent results about genetic and metabolic mechanisms that regulate its activity. These biochemical principles are discussed in the context of PPP deficiencies causing metabolic disease and the role of this pathway in biotechnology, bacterial and parasite infections, neurons, stem cell potency and cancer metabolism.

The metabolic significance of octulose phosphates in the photosynthetic carbon reduction cycle in spinach

14C-Labelled octulose phosphates were formed during photosynthetic 14CO2 fixation and were measured in spinach leaves and chloroplasts. Because mono- and bisphosphates of D: -glycero- D: -ido-octulose are the active 8-carbon ketosugar intermediates of the L-type pentose pathway, it was proposed that they may also be reactants in a modified Calvin-Benson-Bassham pathway reaction scheme. This investigation therefore initially focussed only on the ido-epimer of the octulose phosphates even though 14C-labelled D: -glycero- D: -altro-octulose mono- and bisphosphates were also identified in chloroplasts and leaves. 14CO2 predominantly labelled positions 5 and 6 of D: -glycero- D: -ido-octulose 1,8-P2 consistent with labelling predictions of the modified scheme. The kinetics of 14CO2 incorporation into ido-octulose was similar to its incorporation into some traditional intermediates of the path of carbon, while subsequent exposure to 12CO2 rapidly displaced the 14C isotope label from octulose with the same kinetics of label loss as some of the confirmed Calvin pathway intermediates. This is consistent with octulose phosphates having the role of cyclic intermediates rather than synthesized storage products. (Storage products don't rapidly exchange isotopically labelled carbons with unlabelled CO2.)A spinach chloroplast extract, designated stromal enzyme preparation (SEP), catalysed and was used to measure rates of CO(2) assimilation with Calvin cycle intermediates and octulose and arabinose phosphates. Only pentose (but not arabinose) phosphates and sedoheptulose 7-phosphate supported CO2 fixation at rates in excess of 120 micromol h(-1) mg(-1) Chl. Rates for octulose, sedoheptulose and fructose bisphosphates, octulose, hexose and triose monophosphates were all notably less than the above rate and arabinose 5-phosphate was inactive. Altro-octulose phosphates were more active than phosphate esters of the ido-epimer. The modified scheme proposed a specific phosphotransferase and SEP unequivocally catalysed reversible phosphate transfer between sedoheptulose bisphosphate and D: -glycero- D: -ido-octulose 8-phosphate. It was also initially hypothesized that arabinose 5-phosphate, an L-Type pentose pathway reactant, may have a role in a modified Calvin pathway. Arabinose 5-phosphate is present in spinach chloroplasts and leaves. Radiochromatography showed that 14C-arabinose 5-phosphate with SEP, but only in the presence of an excess of unlabelled ribose 5-phosphate, lightly labelled ribulose 5-phosphate and more heavily labelled hexose and sedoheptulose mono- and bisphosphates. However, failure to demonstrate any CO2 fixation by arabinose 5-phosphate as sole substrate suggested that the above labelling may have no metabolic significance. Despite this arabinose and ribose 5-phosphates are shown to exhibit active roles as enzyme co-factors in transaldolase and aldolase exchange reactions that catalyse the epimeric interconversions of the phosphate esters of ido- and altro-octulose. Arabinose 5-phosphate is presented as playing this role in a New Reaction Scheme for the path of carbon, where it is concluded that slow reacting ido-octulose 1,8 bisphosphate has no role. The more reactive altro-octulose phosphates, which are independent of the need for phosphotransferase processing, are presented as intermediates in the new scheme. Moreover, using the estimates of phosphotransferase activity with altro-octulose monophosphate as substrate allowed calculation of the contributions of the new scheme, that ranged from 11% based on the intact chloroplast carboxylation rate to 80% using the carboxylation rate required for the support of octulose phosphate synthesis and its role in the phosphotransferase reaction.

Comparison of computer simulations of the F-type and L-type non-oxidative hexose monophosphate shunts with 31P-NMR experimental data from human erythrocytes

Mathematical modelling was used to predict the behaviour of the two most favoured schemes for the operation of the non-oxidative hexose monophosphate shunt (HMS), the F-type and the L-type pathways. The models simulate the time courses of sugar-phosphate concentrations when various substrates are metabolized via each pathway. A 31P-NMR technique, with which to observe time courses of concentrations of sugar phosphates in a human red cell lysate, was developed. The accuracy of each hypothesised scheme was then evaluated by comparing predicted with observed data. The results were more consistent with time courses of sugar-phosphate levels predicted by the F-type (classical) pathway than those predicted by the L-type model. However, the accumulation of sedoheptulose 1,7-bisphosphate when a haemolysate was incubated with ribose 5-phosphated showed that the F-type pathway is not a complete description of the system of reactions. Transaldolase was demonstrated to be essential for the normal metabolism of sugar phosphates by haemolysates. The effects of the heat-inactivation of transaldolase on the metabolism of sugar phosphates were accurately predicted by the F-type model. The relevance of attempting to describe the reaction of the non-oxidative HMS as a distinct 'pathway' or 'cycle' is discussed.

Why the L-type pentose pathway does not function in liver

1. The classical pentose and not the L-type pathway functions in liver (Rognstad et al., 1982; Landau and Wood, 1983a; Landau, 1985; Scofield et al., 1985b). 2. It seems necessary to summarize again the reasons for this conclusion because of a recent review by Williams and his coworkers in this Journal (Williams et al., 1987).

Rapid methods for the high yield synthesis of carbon-13 enriched intermediates of the pentose-phosphate pathway

Methods for the synthesis of carbon-13 enriched substrates, intermediates and products of the pentose-phosphate pathway, viz. ribose, arabinose, xylulose and ribulose 5-phosphates, sedoheptulose mono- and bisphosphates, octulose (both the ido- and altro-epimers) mono- and bisphosphates, are described. The procedure of the classical Kiliani synthesis was adopted for the preparation of the two starting compounds, [1-13C]ribose and [1-13C]arabinose 5-phosphates. Using these initial reactants and enzymic methods involving the group-transferring enzymes, transketolase, aldolase and transaldolase, a variety of specifically 13C-labelled five-, six-, seven- and eight-carbon sugar phosphates were synthesized in high yield and purity. The isolation and authenticity of each of the 13C-labelled sugars were established by column, paper and thin layer chromatographic methods and specific enzymic assays. The purity and positional isotopic analysis of these sugar-P's were confirmed by 13C-NMR spectroscopy. These specifically 13C-enriched compounds are required for enzymatic, mechanistic and quantitative investigations of pentose-pathway reactions in animal, plant and tumour tissues in vitro and in vivo.

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.

Identification and measurement of D-glycero D-ido octulose 1,8-bisphosphate: D-altro-heptulose 7-phosphotransferase enzyme in tissues with L-type pentose phosphate pathway activity

The enzyme D-glycero D-ido octulose 1,8-bisphosphate:D-altro-heptulose 7-phosphotransferase (abbreviated to phosphotransferase, PT) catalyses the transfer of the phosphate ester group at C-1 between altro-heptulose (sedoheptulose) and octulose phosphate intermediates of the L-type pentose pathway. Using synthetically prepared and 14C-labelled octulose mono- and bisphosphates, two methods are described for the measurement of the catalytic capacity of the PT reaction operating in both the "forward" and "reverse" modes of L-type pentose pathway operation. PT activity was found in normal, regenerating and foetal rat liver, rat heart, rat epididymal fat pad, rat kidney, brain and skeletal muscle, extracts of C. fusca, pea leaf and a variety of tumour tissues. The highest activity of the enzyme was found in the neoplasms. The Michaelian kinetic constants, temperature and pH optima for the reaction of the enzyme from rat liver together with an assortment of its substrate specificities have been determined. Vanadate anion was found to inhibit the enzyme and the pattern of inhibition suggests that the PT may act by a sequential mechanism. Neither arabinose 5-phosphate nor inorganic phosphate showed any effect on the catalytic activity of the PT enzyme in liver.