Genomic organization and biochemistry of the ribulose monophosphate pathway and its application in biotechnology

3-Hexulose-6-phosphate synthase (HPS) and 6-phospho-3-hexuloisomerase (PHI) are key enzymes catalyzing exergonic reactions of the formaldehyde-fixing reaction and the isomerization of sugar phosphate in the ribulose monophosphate (RuMP) pathway. This pathway, which was originally found in methylotrophic bacteria, is now recognized to be widespread in prokaryotes and has been shown to be involved not only in formaldehyde fixation and detoxification but also in pentose phosphate biosynthesis. In this review, we describe the genomic organization and regulation of the genes of the RuMP pathway and then discuss the physiological roles of this pathway in prokaryotes. We further describe the biochemical properties of HPS and PHI. Heterologous expression of HPS and PHI in various organisms allows them to metabolize and detoxify formaldehyde, and we also review recent progress in such applications in biotechnology.     

The physiological role of the ribulose monophosphate pathway in bacteria and archaea

3-Hexulose-6-phosphate synthase (HPS) and 6-phospho-3-hexuloisomerase (PHI) are the key enzymes of the ribulose monophosphate pathway. This pathway, which was originally found in methylotrophic bacteria, is now recognized as a widespread prokaryotic pathway involved in formaldehyde fixation and detoxification. Recent progress, involving biochemical and genetic approaches in elucidating the physiological functions of HPS and PHI in methylotrophic as well as non-methylotrophic bacteria are described in this review. HPS and PHI orthologs are also found in a variety of archaeal strains. Some archaeal HPS orthologs are fused with other genes to form single ORF (e.g., the hps-phi gene of Pyrococcus spp. and the faeB-hpsB gene of Methanosarcina spp). These fused gene products exhibit functions corresponding to the individual enzyme activities, and are more efficient than equivalent systems made up of discrete enzymes. Recently, a novel metabolic function for HPS and PHI has been proposed in which these enzymes catalyze the reverse reaction for the biosynthesis of pentose phosphate in some archaeal strains. Thus the enzyme system plays a different role in bacteria and archaea by catalyzing the forward and reverse reactions respectively.     

细菌的核酮糖单磷酸途径与甲醛同化作用

核酮糖单磷酸途径最初在甲基营养菌中发现,现在被认为是在细菌中广泛存在的和甲醛同化作用及脱毒相关的一条途径,该途径的关键酶是6-磷酸己酮糖合成酶和6-磷酸己酮糖异构酶。文章将介绍来源于各种细菌的核酮糖单磷酸途径的生理作用及其两个关键酶基因的组织结构、表达调控机制与应用前景。     

Biosynthesis of ribose-5-phosphate and erythrose-4-phosphate in archaea: a phylogenetic analysis of archaeal genomes

A phylogenetic analysis of the genes encoding enzymes in the pentose phosphate pathway (PPP), the ribulose monophosphate (RuMP) pathway, and the chorismate pathway of aromatic amino acid biosynthesis, employing data from 13 complete archaeal genomes, provides a potential explanation for the enigmatic phylogenetic patterns of the PPP genes in archaea. Genomic and biochemical evidence suggests that three archaeal species (Methanocaldococcus jannaschii, Thermoplasma acidophilum and Thermoplasma volcanium) produce ribose-5-phosphate via the nonoxidative PPP (NOPPP), whereas nine species apparently lack an NOPPP but may employ a reverse RuMP pathway for pentose synthesis. One species (Halobacterium sp. NRC-1) lacks both the NOPPP and the RuMP pathway but may possess a modified oxidative PPP (OPPP), the details of which are not yet known. The presence of transketolase in several archaeal species that are missing the other two NOPPP genes can be explained by the existence of differing requirements for erythrose-4-phosphate (E4P) among archaea: six species use transketolase to make E4P as a precursor to aromatic amino acids, six species apparently have an alternate biosynthetic pathway and may not require the ability to make E4P, and one species (Pyrococcus horikoshii) probably does not synthesize aromatic amino acids at all.     

The archaeon Pyrococcus horikoshii possesses a bifunctional enzyme for formaldehyde fixation via the ribulose monophosphate pathway

Pyrococcus horikoshii OT3, a hyperthermophilic and anaerobic archaeon, was found to have an open reading frame (PH1938) whose deduced amino acid sequence of the N-terminal and C-terminal halves showed significant similarity to two key enzymes of the ribulose monophosphate pathway for formaldehyde fixation in methylotrophic bacteria, 3-hexulose-6-phosphate synthase (HPS) and 6-phospho-3-hexuloisomerase (PHI), respectively. The organism constitutively produced the encoded protein and exhibited activity of the sequential HPS- and PHI-mediated reactions in a particulate fraction. The full-length gene encoding the hybrid enzyme, the sequence corresponding to the HPS region, and the sequence corresponding to the PHI region were expressed in Escherichia coli and were found to produce active enzymes, rHps-Phi, rHps, or rPhi, respectively. Purified rHps-Phi and rHps were found to be active at the growth temperatures of the parent strain, but purified rPhi exhibited significant susceptibility to heat, suggesting that thermostability of the PHI moiety of the bifunctional enzyme (rHps-Phi) resulted from fusion with HPS. The bifunctional enzyme catalyzed the sequential reaction much more efficiently than a mixture of rHps and rPhi. These and other biochemical characterizations of the PH1938 gene product suggest that the ribulose monophosphate pathway plays a significant role in the archaeon under extreme environmental conditions.     

Bacillus subtilis yckG and yckF encode two key enzymes of the ribulose monophosphate pathway used by methylotrophs, and yckH is required for their expression

The ribulose monophosphate (RuMP) pathway is one of the metabolic pathways for the synthesis of compounds containing carbon-carbon bonds from one-carbon units and is found in many methane- and methanol-utilizing bacteria, which are known as methylotrophs. The characteristic enzymes of this pathway are 3-hexulose-6-phosphate synthase (HPS) and 6-phospho-3-hexuloisomerase (PHI), neither of which was thought to exist outside methylotrophs. However, the presumed yckG gene product (YckG) of Bacillus subtilis shows a primary structure similar to that of methylotroph HPS (F. Kunst et al., Nature 390:249-256, 1997). We have also investigated the sequence similarity between the yckF gene product (YckF) and methylotroph PHI (Y. Sakai, R. Mitsui, Y. Katayama, H. Yanase, and N. Kato, FEMS Microbiol. Lett. 176:125-130, 1999) and found that the yckG and yckF genes of B. subtilis express enzymatic activities of HPS and PHI, respectively. Both of these activities were concomitantly induced in B. subtilis by formaldehyde, with induction showing dependence on the yckH gene, but were not induced by methanol, formate, or methylamine. Disruption of either gene caused moderate sensitivity to formaldehyde, suggesting that these enzymes may act as a detoxification system for formaldehyde in B. subtilis. In conclusion, we found an active yckG (for HPS)-yckF (for PHI) gene structure (now named hxlA-hxlB) in a nonmethylotroph, B. subtilis, which inherently preserves the RuMP pathway.     

The carbon assimilation pathways of Methylococcus capsulatus, Pseudomonas methanica and Methylosinus trichosporium (OB3B) during growth on methane

d-arabino-3-Hexulose 6-phosphate was prepared by condensation of formaldehyde with ribulose 5-phosphate in the presence of 3-hexulose phosphate synthase from methane-grown Methylococcus capsulatus. The 3-hexulose phosphate was unstable in solutions of pH greater than 3, giving a mixture of products in which, after dephosphorylation, allulose and fructose were detected. A complete conversion of d-ribulose 5-phosphate and formaldehyde into d-fructose 6-phosphate was demonstrated in the presence of 3-hexulose phosphate synthase and phospho-3-hexuloisomerase (prepared from methane-grown M. capsulatus). d-Allulose 6-phosphate was prepared from d-allose by way of d-allose 6-phosphate. No evidence was found for its metabolism by extracts of M. capsulatus, thus eliminating it as an intermediate in the carbon assimilation process of this organism. A survey was made of the enzymes involved in the regeneration of pentose phosphate during C(1) assimilation via a modified pentose phosphate cycle. On the basis of the presence of the necessary enzymes, two alternative routes for cleavage of fructose 6-phosphate are suggested, one route involves fructose diphosphate aldolase and the other 6-phospho-2-keto-3-deoxygluconate aldolase. A detailed formulation of the complete ribulose monophosphate cycle of formaldehyde fixation is presented. The energy requirements for carbon assimilation by this cycle are compared with those for the serine pathway and the ribulose diphosphate cycle of carbon dioxide fixation. A cyclic scheme for oxidation of formaldehyde via 6-phosphogluconate is suggested.     

Crystal structure of MJ1247 protein from M. jannaschii at 2.0 A resolution infers a molecular function of 3-hexulose-6-phosphate isomerase

The crystal structure of the hypothetical protein MJ1247 from Methanococccus jannaschii at 2 A resolution, a detailed sequence analysis, and biochemical assays infer its molecular function to be 3-hexulose-6-phosphate isomerase (PHI). In the dissimilatory ribulose monophosphate (RuMP) cycle, ribulose-5-phosphate is coupled to formaldehyde by the 3-hexulose-6-phosphate synthase (HPS), yielding hexulose-6-phosphate, which is then isomerized to fructose-6-phosphate by the enzyme 3-hexulose-6-phosphate isomerase. MJ1247 is an alpha/beta structure consisting of a five-stranded parallel beta sheet flanked on both sides by alpha helices, forming a three-layered alpha-beta-alpha sandwich. The fold represents the nucleotide binding motif of a flavodoxin type. MJ1247 is a tetramer in the crystal and in solution and each monomer has a folding similar to the isomerase domain of glucosamine-6-phosphate synthase (GlmS).     

Evidence for a pentose phosphate pathway in Helicobacter pylori

Abstract Evidence for the presence of enzymes of the pentose phosphate pathway in Helicobacter pylori was obtained using ³¹P nuclear magnetic resonance spectroscopy. Activities of enzymes which are part of the oxidative and non-oxidative phases of the pathway were observed directly in incubations of bacterial lysates with pathway intermediates. Generation of NADPH and 6-phosphogluconate from NADP⁺ and glucose 6-phosphate indicated the presence of glucose 6-phosphate dehydrogenase and 6-phosphogluconolactonase. Reduction of NADP⁺ with production of ribulose 5-phosphate from 6-phosphogluconate revealed 6-phosphogluconate dehydrogenase activity. Phosphopentose isomerase and transketolase activities were observed in incubations containing ribulose 5-phosphate and xylulose 5-phosphate, respectively. The formation of erythrose 4-phosphate from xylulose 5-phosphate and ribose 5-phosphate suggested the presence of transaldolase. The activities of this enzyme and triosephosphate isomerase were observed directly in incubations of bacterial lysates with dihydroxyacetone phosphate and sedoheptulose 7-phosphate. Glucose-6-phosphate isomerase activity was measured in incubations with fructos 6-phosphate. The presence of these enzymes in H. pylori suggested the existence of a pentose phosphate pathway in the bacterium, possibly as a mechanism to provide NADPH for reductive biosynthesis and ribose 5-phosphate for synthesis of nucleic acids.     

Ribose-5-phosphate biosynthesis in Methanocaldococcus jannaschii occurs in the absence of a pentose-phosphate pathway

Recent work has raised a question as to the involvement of erythrose-4-phosphate, a product of the pentose phosphate pathway, in the metabolism of the methanogenic archaea (R. H. White, Biochemistry 43:7618-7627, 2004). To address the possible absence of erythrose-4-phosphate in Methanocaldococcus jannaschii, we have assayed cell extracts of this methanogen for the presence of this and other intermediates in the pentose phosphate pathway and have determined and compared the labeling patterns of sugar phosphates derived metabolically from [6,6-2H2]- and [U-13C]-labeled glucose-6-phosphate incubated with cell extracts. The results of this work have established the absence of pentose phosphate pathway intermediates erythrose-4-phosphate, xylose-5-phosphate, and sedoheptulose-7-phosphate in these cells and the presence of D-arabino-3-hexulose-6-phosphate, an intermediate in the ribulose monophosphate pathway. The labeling of the D-ara-bino-3-hexulose-6-phosphate, as well as the other sugar-Ps, indicates that this hexose-6-phosphate was the precursor to ribulose-5-phosphate that in turn was converted into ribose-5-phosphate by ribose-5-phosphate isomerase. Additional work has demonstrated that ribulose-5-phosphate is derived by the loss of formaldehyde from D-arabino-3-hexulose-6-phosphate, catalyzed by the protein product of the MJ1447 gene.     

Subcellular distribution of enzymes of the oxidative pentose phosphate pathway in root and leaf tissues

The subcellular distribution of enzymes of the oxidative pentose phosphate pathway was studied in plants. Root and leaf tissues from several species were separated by differential centrifugation into plastidic and cytosolic fractions. In all tissues studied, glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were found in both plastidic and cytosolic compartments. In maize and pea root, and spinach and pea leaf, the non-oxidative enzymes of the pentose phosphate pathway (transaldolase, transketolase, ribose 5-phosphate isomerase, ribulose 5-phosphate 3-epimerase) appear to be restricted to the plastid. In tobacco leaf and root, however, the non-oxidative enzymes were found in the cytosolic as well as the plastidic compartments. In the absence of ribose 5-phosphate isomerase and ribulose 5-phosphate 3-epimerase in the cytosol, the product of the oxidative limb of the pathway (ribulose 5-phosphate) must be transported into a compartment capable of utilizing it. Ribulose 5-phosphate was supplied to isolated intact pea root plastids and was shown to be capable of supporting nitrite reduction. The kinetics of ribulose 5-phosphate-driven nitrite reduction in isolated pea root plastids suggested that the metabolite was translocated across the plastid envelope in a carrier-mediated transport process, indicating the presence of a translocator capable of transporting pentose phosphates.Keywords: Pentose phosphate, subcellular, plastid, ribulose 5-phosphate, compartmentation      

Methanol metabolism in thermotolerant methylotrophic Bacillus strains involving a novel catabolic NAD-dependent methanol dehydrogenase as a key enzyme

The enzymology of methanol utilization in thermotolerant methylotrophic Bacillus strains was investigated. In all strains an immunologically related NAD-dependent methanol dehydrogenase was involved in the initial oxidation of methanol. In cells of Bacillus sp. C1 grown under methanol-limiting conditions this enzyme constituted a high percentage of total soluble protein. The methanol dehydrogenase from this organism was purified to homogeneity and characterized. In cell-free extracts the enzyme displayed biphasic kinetics towards methanol, with apparent K _m values of 3.8 and 166 mM. Carbon assimilation was by way of the fructose-1,6-bisphosphate aldolase cleavage and transketolase/transaldolase rearrangement variant of the RuMP cycle of formaldehyde fixation. The key enzymes of the RuMP cycle, hexulose-6-phosphate synthase (HPS) and hexulose-6-phosphate isomerase (HPI), were present at very high levels of activity. Failure of whole cells to oxidize formate, and the absence of formaldehyde-and formate dehydrogenases indicated the operation of a non-linear oxidation sequence for formaldehyde via HPS. A comparison of the levels of methanol dehydrogenase and HPS in cells of Bacillus sp. C1 grown on methanol and glucose suggested that the synthesis of these enzymes is not under coordinate control.Abbreviations RuMP ribulose monophosphate - HPS hexulose-6-phosphate synthase - HPI hexulose-6-phosphate isomerase - MDH methanol dehydrogenase - ADH acohol dehydrogenase - PQQ pyrroloquinoline, quinone - DTT dithiothreitol - NBT nitrobluetetrazolium - PMS phenazine methosulphate - DCPIP dichlorophenol indophenol     

Over-expression of glucose dehydrogenase improves cell growth and riboflavin production in Bacillus subtilis

Ribulose 5-phosphate is a precursor for riboflavin biosynthesis. Alteration of carbon flow into the pentose phosphate pathway will affect the availability of ribulose 5-phosphate and the riboflavin yield. We have modulated carbon flow in Bacillus subtilis through the gluconate bypass by over-expression of glucose dehydrogenase under the control of the constitutively expressed P43 promoter. Over-expression of glucose dehydrogenase resulted in low acid production (acetate and pyruvate). The substantial reduction in acid production is accompanied by increased riboflavin production and an increased rate of growth while glucose consumption remained unchanged. Metabolic analysis indicated that over-expression of glucose dehydrogenase increased intracellular pool of ribulose 5-phosphate. The high concentrations of ribulose 5-phosphate could explain the increased riboflavin production.     

Carbon metabolism of chloroplasts in the dark: Oxidative pentose phosphate cycle versus glycolytic pathway

The conversion of U-labelled [¹⁴C]glucose-6-phosphate into other products by a soluble fraction of lysed spinach chloroplasts has been studied. It was found that both an oxidative pentose phosphate cycle and a glycolytic reaction sequence occur in this fraction. The formation of bisphosphates and of triose phosphates was ATP-dependent and occurred mainly via a glycolytic reaction sequence including a phosphofructokinase step. The conversion, of glucose-6-phosphate via the oxidative pentose phosphate cycle stopped with the formation of pentose monophosphates. This was found not to be because of a lack in transaldolase (or transketolase) activity, but because of the high concentration ratios of hexose monophosphate/pentose monophosphate used in our experiments for simulating the conditions in whole chloroplasts in the dark. Some regulatory properties of both the oxidative pentose phosphate cycle and of the glycolytic pathway were studied.Abbreviations DHAP dihydroxyacetone phosphate - GAP 3-phosphoglyceraldehyde - PGA 3-phosphoglycerate - HMP hexose monophosphates - including F6P fructose-6-phosphate - G6P glucose-6-phosphate - GIP glucose-1-phosphate - 6-PGL phosphogluconate - PMP pentose monophosphates - including R5P ribose-5-phosphate - Ru5P ribulose-5-phosphate - X5P xylulose-5-phosphate - E4P erythrose-4-phosphate - S7P sedoheptulose-7-phosphate - FBP fructose-1,6-bisphosphate - SBP sedoheptulose-1,7-bisphosphate - RuBP ribulose-1,5-bisphosphate     

Regulation of methanol metabolism in the facultative methylotroph Nocardia sp. 239 during growth on mixed substrates in batch- and continuous cultures

The regulation of methanol metabolism in Nocardia sp. 239 was investigated. Growth on mixtures of glucose or acetate plus methanol in batch cultures resulted in simultaneous utilization of the substrates. The presence of glucose, but not of acetate, repressed synthesis of the ribulose monophosphate (RuMP) cycle enzymes hexulose-6-phosphate synthase (HPS) and hexulose-6-phosphate isomerase (HPI), and methanol was used as an energy source only. Comparable results were obtained following addition of formaldehyde (fed-batch system) to a culture growing on glucose. The synthesis of the methanol dissimilatory and assimilatory enzymes in Nocardia sp. 239 thus appears to be controlled differently. Methanol and/or formaldehyde induce the synthesis of these enzymes, but under carbon-excess conditions their inducing effect on HPS and HPI synthesis is completely overruled by glucose, or metabolites derived from it. Repression of the synthesis of these RuMP cycle enzymes was of minor importance under carbon- and energy-limiting conditions in chemostat cultures. Addition of a pulse of glucose to a formaldehyde-limited (2.5 mmol l^–1 h^–1) fed-batch culture resulted in a decrease in the levels of several enzymes of methanol metabolism (including HPI), whereas the HPS levels remained relatively constant. Increasing HPS/HPI activity ratios were also observed with increasing growth rates in formaldehyde-limited chemostat cultures. The data indicate that additional mechanisms, the identity of which remains to be elucidated, are involved in controlling the levels of these C₁-specific enzymes in Nocardia sp. 239.Abbreviations HPS hexulose-6-phosphate synthase - HPI hexulose-6-phosphate isomerase - RuMP ribulose monophosphate - FBP fructose-1,6-bisphosphate - PFK 6-phosphofructokinase     

The pentose phosphate pathway of glucose metabolism. Hormonal and dietary control of the oxidative and non-oxidative reactions of the cycle in liver

1. Measurements were made of the non-oxidative reactions of the pentose phosphate cycle in liver (transketolase, transaldolase, ribulose 5-phosphate epimerase and ribose 5-phosphate isomerase activities) in a variety of hormonal and nutritional conditions. In addition, glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase activities were measured for comparison with the oxidative reactions of the cycle; hexokinase, glucokinase and phosphoglucose isomerase activities were also included. Starvation for 2 days caused significant lowering of activity of all the enzymes of the pentose phosphate cycle based on activity in the whole liver. Re-feeding with a high-carbohydrate diet restored all the enzyme activities to the range of the control values with the exception of that of glucose 6-phosphate dehydrogenase, which showed the well-known ;overshoot' effect. Re-feeding with a high-fat diet also restored the activities of all the enzymes of the pentose phosphate cycle and of hexokinase; glucokinase activity alone remained unchanged. Expressed as units/g. of liver or units/mg. of protein hexokinase, glucose 6-phosphate dehydrogenase, transketolase and pentose phosphate isomerase activities were unchanged by starvation; both 6-phosphogluconate dehydrogenase and ribulose 5-phosphate epimerase activities decreased faster than the liver weight or protein content. 2. Alloxan-diabetes resulted in a decrease of approx. 30-40% in the activities of 6-phosphogluconate dehydrogenase, ribose 5-phosphate isomerase, ribulose 5-phosphate epimerase and transketolase; in contrast with this glucose 6-phosphate dehydrogenase, transaldolase and phosphoglucose isomerase activities were unchanged. Treatment of alloxan-diabetic rats with protamine-zinc-insulin for 3 days caused a very marked increase to above normal levels of activity in all the enzymes of the pentose phosphate pathway except ribulose 5-phosphate epimerase, which was restored to the control value. Hexokinase activity was also raised by this treatment. After 7 days treatment of alloxan-diabetic rats with protamine-zinc-insulin the enzyme activities returned towards the control values. 3. In adrenalectomized rats the two most important changes were the rise in hexokinase activity and the fall in transketolase activity; in addition, ribulose 5-phosphate epimerase activity was also decreased. These effects were reversed by cortisone treatment. In addition, in cortisone-treated adrenalectomized rats glucokinase activity was significantly lower than the control value. 4. In thyroidectomized rats both ribose 5-phosphate isomerase and transketolase activities were decreased; in contrast with this transaldolase activity did not change significantly. Hypophysectomy caused a 50% fall in transketolase activity that was partially reversed by treatment with thyroxine and almost fully reversed by treatment with growth hormone for 8 days. 5. The results are discussed in relation to the hormonal control of the non-oxidative reactions of the pentose phosphate cycle, the marked changes in transketolase activity being particularly outstanding.     

The pentose phosphate pathway of glucose metabolism. Hormonal and dietary control of the oxidative and non-oxidative reactions and related enzymes of the cycle in adipose tissue

1. Measurements were made of the activities of the enzymes of the pentose phosphate pathway concerned in both the oxidative (glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase) and the non-oxidative (ribose 5-phosphate isomerase, ribulose 5-phosphate epimerase, transketolase and transaldolase) reactions of this pathway, together with hexokinase and phosphoglucose isomerase, in adipose tissue in a variety of nutritional and hormonal conditions. 2. Starvation for 2 days caused a significant decrease in the activities of all the enzymes of the pentose phosphate pathway, with the exception of glucose 6-phosphate dehydrogenase, when expressed as activity/2 fat-pads; only the activities of ribose 5-phosphate isomerase and ribulose 5-phosphate epimerase were significantly decreased on the basis of activity/mg. of protein. Re-feeding with a high-carbohydrate or high-fat diet for 3 days restored the activity of all the enzymes of the pentose phosphate pathway to the range of the control values, with the exception of transketolase, which showed a marked ;overshoot' in rats re-fed with carbohydrate. Starvation for 3 days caused a marked decrease in the activities of glucose 6-phosphate dehydrogenase and transketolase. 3. On the basis of activity/two fat-pads, alloxan-diabetes caused a marked decrease, to about half the control value, in the activities of all the enzymes concerned in the pentose phosphate pathway, transketolase showing the smallest decrease; hexokinase and phosphoglucose isomerase activities were also decreased. Treatment with insulin for 3 and 7 days raised the activities to normal or supranormal values, transketolase showing the most marked ;overshoot' effect. On the basis of activity/mg. of protein the activity of none of the enzymes was significantly decreased in alloxan-diabetes; transketolase and transaldolase activities were raised above the control values. With insulin treatment for 3 or 7 days the activities of all the enzymes were significantly increased, except that of ribulose 5-phosphate epimerase at the shorter time-interval. Glucagon treatment did not alter any of the enzyme activities expressed on either basis. 4. Thyroidectomy caused a decrease of 30-40% in the activities of enzymes of the pentose phosphate pathway, except for transketolase activity, which fell to 50% of the control value. Little change occurred in adipose-tissue weight or protein content. 5. Adrenalectomy caused a decrease of 40% in the activity of glucose 6-phosphate dehydrogenase and of 20-30% in the activities of the remaining enzymes of the pentose phosphate pathway; hexokinase activity was also decreased. Treatment with cortisone for 3 days did not significantly raise the activity from that found in adrenalectomized rats. Treatment of normal rats with high doses of cortisone had no significant effect on the activities of the enzymes of the pentose phosphate pathway in adipose tissue. 6. The changes in enzyme activities are discussed in relation to: (a) the concept of constant-proportion groups of enzymes; (b) the known changes in the flux of glucose through alternative metabolic pathways; (c) the pattern of change found in liver with similar hormonal and dietary conditions.     

植物戊糖磷酸途径及其两个关键酶的研究进展

戊糖磷酸途径是植物体中糖代谢的重要途径,主要生理功能是产生供还原性生物合成需要的NADPH,可供核酸代谢的磷酸戊糖以及一些中间产物可参与氨基酸合成和脂肪酸合成等.葡萄糖-6-磷酸脱氢酶和6-磷酸葡萄糖酸脱氢酶是戊糖磷酸途径的两个关键酶,广泛的分布于高等植物的胞质和质体中.本文综述了植物戊糖磷酸途径及其两个关键酶的分子生物学的研究进展,讨论了该途径在植物生长发育和环境胁迫应答中的作用.