An Hg-sensitive channel mediates the diffusional component of glucose transport in olive cells

2-NBDG Uptake in Gossypium hirsutum in vitro ovules: exploring tissue-specific accumulation and its impact on hexokinase-mediated glycolysis regulation

Fluorescent glucose derivatives are valuable tools as glucose analogs in plant research to explore metabolic pathways, study enzyme activity, and investigate cellular processes related to glucose metabolism and sugar transport. They allow visualization and tracking of glucose uptake, its utilization, and distribution within plant cells and tissues. This study investigates the phenotypic and metabolic impact of the exogenously fed glucose derivative, 2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-2-deoxyglucose) (2-NBDG) on the fibers of Gossypium hirsutum (Upland cotton) ovule in vitro cultures. The presence of 2-NBDG in the culture medium did not lead to macroscopic morphological alterations in ovule and fiber development or to the acquisition of fluorescence or yellow coloration. Confocal laser scanning microscope imaging and chromatographic analysis of cotton ovules' outer rim cross-sections showed that the 2-NBDG is transported from the extracellular space and accumulated inside some outer integument cells, epidermal cells, and fertilized epidermal cells (fibers), but is not incorporated into the cell walls. Untargeted metabolic profiling of the fibers revealed significant changes in the relative levels of metabolites involved in glycolysis and upregulation of alternative energy-related pathways. To provide biochemical and structural evidence for the observed downregulation of glycolysis pathways in the fibers containing 2-NBDG, kinetics analysis and docking simulations were performed on hexokinase from G. hirsutum (GhHxk). Notably, the catalytic activity of heterologously expressed recombinant active GhHxk exhibited a five-fold decrease in reaction rates compared to D-glucose. Furthermore, GhHxk exhibited a linear kinetic behavior in the presence of 2-NBDG instead of the Michaelis-Menten kinetics found for D-glucose. Docking simulations suggested that 2-NBDG interacts with a distinct binding site of GhHxk9, possibly inducing a conformational change. These results highlight the importance of considering fluorescent glucose derivatives as ready-to-use analogs for tracking glucose-related biological processes. However, a direct comparison between their mode of action and its extrapolation into biochemical considerations should go beyond microscopic inspection and include complementary analytical techniques.

Uptake of fluorescent D- and L-glucose analogues, 2-NBDG and 2-NBDLG, into human osteosarcoma U2OS cells in a phloretin-inhibitable manner

Mammalian cells take in d-glucose as an essential fuel as well as a carbon source. In contrast, l-glucose, the mirror image isomer of d-glucose, has been considered merely as a non-transportable/non-metabolizable control for d-glucose. We have shown that 2-[N-(7-Nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxy-D-glucose (2-NBDG), a d-glucose analogue combining a fluorophore NBD at the C-2 position, is useful as a tracer for monitoring d-glucose uptake through glucose transporters (GLUTs) into mammalian cells. To more precisely evaluate the stereoselectivity of 2-NBDG uptake, we developed an l-glucose analogue 2-NBDLG, the mirror-image isomer of 2-NBDG. Interestingly, 2-NBDLG was taken up into mouse insulinoma MIN6 cells showing nuclear heterogeneity, a cytological feature of malignancy, while remaining MIN6 cells only exhibited a trace amount of 2-NBDLG uptake. The 2-NBDLG uptake into MIN6 cells was abolished by phloretin, but persisted under blockade of major mammalian glucose transporters. Unfortunately, however, no such uptake could be detected in other tumor cell lines. Here we demonstrate that human osteosarcoma U2OS cells take in 2-NBDLG in a phloretin-inhibitable manner. The uptake of 2-NBDG, and not that of 2-NBDLG, into U2OS cells was significantly inhibited by cytochalasin B, a potent GLUT inhibitor. Phloretin, but neither phlorizin, an inhibitor of sodium-glucose cotransporter (SGLT), nor a large amount of d/l-glucose, blocked the 2-NBDLG uptake. These results suggest that a phloretin-inhibitable, non-GLUT/non-SGLT, possibly non-transporter-mediated yet unidentified mechanism participates in the uptake of the fluorescent l-glucose analogue in two very different tumor cells, the mouse insulinoma and the human osteosarcoma cells.

L-Glucose: Another Path to Cancer Cells

Cancerous tumors comprise cells showing metabolic heterogeneity. Among numerous efforts to understand this property, little attention has been paid to the possibility that cancer cells take up and utilize otherwise unusable substrates as fuel. Here we discuss this issue by focusing on l-glucose, the mirror image isomer of naturally occurring d-glucose; l-glucose is an unmetabolizable sugar except in some bacteria. By combining relatively small fluorophores with l-glucose, we generated fluorescence-emitting l-glucose tracers (fLGs). To our surprise, 2-NBDLG, one of these fLGs, which we thought to be merely a control substrate for the fluorescent d-glucose tracer 2-NBDG, was specifically taken up into tumor cell aggregates (spheroids) that exhibited nuclear heterogeneity, a major cytological feature of malignancy in cancer diagnosis. Changes in mitochondrial activity were also associated with the spheroids taking up fLG. To better understand these phenomena, we review here the Warburg effect as well as key studies regarding glucose uptake. We also discuss tumor heterogeneity involving aberrant uptake of glucose and mitochondrial changes based on the data obtained by fLG. We then consider the use of fLGs as novel markers for visualization and characterization of malignant tumor cells.

Aberrant Uptake of a Fluorescent L-Glucose Analogue (fLG) into Tumor Cells Expressing Malignant Phenotypes

Glucose, one of the most fundamental sugar elements, has either D- or L-conformation. Of these, most cells preferentially take up D-glucose as an essential energy/carbon source. Such stereoselective uptake of glucose has been explored by fluorophore-bearing D- and L-glucose analogues. 2-[N-(7-Nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxy-D-glucose (2-NBDG), the most widely used fluorescent D-glucose analogue, was abundantly taken up into living Escherichia coli cells, whereas no detectable uptake was obtained for 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxy-L-glucose (2-NBDLG), the antipode of 2-NBDG developed as a fluorescent L-glucose analogue (fLG). Interestingly, we found three-dimensionally accumulating tumor cell aggregates taking up 2-NBDLG when they expressed nuclear heterogeneity, one of the major cytological criteria for cells suspected of high-grade malignancy in clinical diagnosis. 2-NBDLG uptake was not detected in aggregates consisting of homogeneous cells and was specifically abolished by phloretin, a broad-spectrum inhibitor against transporters/channels. Preliminary studies have suggested that a combined use of 2-NBDLG, which emits green fluorescence, with 13-[4-[(2-deoxy-D-glucopyranose-2-yl)aminosulfonyl]-2-sulfonatophenyl]-4,5-trimethylene-7,8-trimethylene-1,2,3,4,6,9,10,11-octahydro-4-aza-6-oxa-8-azoniapentacene (2-TRLG), a membrane-impermeable fLG bearing a large red fluorophore, is effective for discriminating malignant tumor from benign cells both in living biopsy specimens endoscopically dissected from patients with early-stage gastric cancer and in ascites fluid of patients with gynecological cancers. Confocal endomicroscopic imaging of a carcinogen-induced cancer in bile duct of hamsters indicated that the fLG uptake pattern well correlated with pathological diagnosis for carcinoma. Safety tests according to Good Laboratory Practice regulations have been successfully completed so far. fLGs are unique fluorescent glucose analogues for identifying and characterizing living cancer cells based on derangements in their transport function.

Uptake of a fluorescent L-glucose derivative 2-NBDLG into three-dimensionally accumulating insulinoma cells in a phloretin-sensitive manner

Of two stereoisomers of glucose, only d- and not l-glucose is abundantly found in nature, being utilized as an essential fuel by most organisms. The uptake of d-glucose into mammalian cells occurs through glucose transporters such as GLUTs, and this process has been effectively monitored by a fluorescent d-glucose derivative 2-[N-(7-Nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxy-d-glucose (2-NBDG) at the single cell level. However, since fluorescence is an arbitrary measure, we have developed a fluorescent analog of l-glucose 2-[N-(7-Nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxy-l-glucose (2-NBDLG), as a negative control substrate for more accurately identifying the stereoselectivity of the uptake. Interestingly, a small portion of mouse insulinoma cells MIN6 abundantly took up 2-NBDLG at a late culture stage (≳10 days in vitro, DIV) when multi-cellular spheroids exhibiting heterogeneous nuclei were formed, whereas no such uptake was detected at an early culture stage (≲6 DIV). The 2-NBDLG uptake was persistently observed in the presence of a GLUT inhibitor cytochalasin B. Neither d- nor l-glucose in 50 mM abolished the uptake. No significant inhibition was detected by inactivating sodium/glucose cotransporters (SGLTs) with Na⁺-free condition. To our surprise, the 2-NBDLG uptake was totally inhibited by phloretin, a broad spectrum inhibitor against transporters/channels including GLUTs and aquaporins. From these, a question might be raised if non-GLUT/non-SGLT pathways participate in the 2-NBDLG uptake into spheroid-forming MIN6 insulinoma. It might also be worthwhile investigating whether 2-NBDLG can be used as a functional probe for detecting cancer, since the nuclear heterogeneity is among critical features of malignancy.

2-Deoxy-2-fluoro-d-glucose metabolism in Arabidopsis thaliana

2-Deoxy-2-fluoro-d-glucose (FDG) is glucose analog routinely used in clinical and animal radiotracer studies to trace glucose uptake but it has rarely been used in plants. Previous studies analyzed FDG translocation and distribution pattern in plants and proposed that FDG could be used as a tracer for photoassimilates in plants. Elucidating FDG metabolism in plants is a crucial aspect for establishing its application as a radiotracer in plant imaging. Here, we describe the metabolic fate of FDG in the model plant species Arabidopsis thaliana. We fed FDG to leaf tissue and analyzed leaf extracts using MS and NMR. On the basis of exact mono-isotopic masses, MS/MS fragmentation, and NMR data, we identified 2-deoxy-2-fluoro-gluconic acid, FDG-6-phosphate, 2-deoxy-2-fluoro-maltose, and uridine-diphosphate-FDG as four major end products of FDG metabolism. Glycolysis and starch degradation seemed to be the important pathways for FDG metabolism. We showed that FDG metabolism in plants is considerably different than animal cells and goes beyond FDG-phosphate as previously presumed.

An update on sugar transport and signalling in grapevine

In addition to their role as a source of reduced carbon, sugars may directly or indirectly control a wide range of activities in plant cells, through transcriptional and post-translational regulation. This control has been studied in detail using Arabidopsis thaliana, where genetic analysis offers many possibilities. Much less is known about perennial woody species. For several years, various aspects of sugar sensing and signalling have been investigated in the grape (Vitis vinifera L.) berry, an organ that accumulates high concentrations of hexoses in the vacuoles of flesh cells. Here we review various aspects of this topic: the molecular basis of sugar transport and its regulation by sugars in grapevine; the functional analysis of several sugar-induced genes; the effects of some biotic and abiotic stresses on the sugar content of the berry; and finally the effects of exogenous sugar supply on the ripening process in field conditions. A picture of complex feedback and multiprocess regulation emerges from these data.

Uptake and phloem transport of glucose-fipronil conjugate in Ricinus communis involve a carrier-mediated mechanism

Some compounds containing glucose are absorbed via the monosaccharide transporters of the plasma membrane. A glucose-fipronil conjugate, N-[3-cyano-1-[2,6-dichloro-4-(trifluoromethyl)phenyl]-4-[(trifluoromethyl)sulfinyl]-1H-pyrazol-5-yl]-1-(β-d-glucopyranosyl)-1H-1,2,3-triazole-4-methanamine (GTF), has been synthesized in our previous work. GTF exhibits moderate phloem mobility in Ricinus communis. In the current paper, we demonstrate that the uptake of GTF by Ricinus seedling cotyledon discs is partly mediated by an active carrier system (K(m)1 = 0.17 mM; V(max)1 = 2.2 nmol cm(-2) h(-1)). Four compounds [d-glucose, sucrose, phloridzin, and carbonyl cyanide m-chlorophenylhydrazone (CCCP)] were examined for their effect on GTF uptake. Phloridzin as well as CCCP markedly inhibit GTF uptake, and d-glucose weakly competes with it. The phloem transport of GTF in Ricinus seedlings is found to involve an active carrier-mediated mechanism that effectively contributes to the GTF phloem loading. The results prove that adding a glucose core is a reasonable and feasible approach to confer phloem mobility to fipronil by utilizing plant monosaccharide transporters.

Insights into Maize LEA proteins: from proteomics to functional approaches

LEA (late embryogenesis abundant) proteins participate in plant stress tolerance responses, but the mechanisms by which protection occurs are not fully understood. In the present work the unfolded proteins from maize dry embryos were analyzed by mass spectrometry. Twenty embryo proteins were identified, and among them 13 corresponded to LEA-type proteins. We selected three major LEA proteins, Emb564, Rab17 and Mlg3, belonging to groups 1, 2 and 3, respectively, and we undertook a comparative study in order to highlight differences among them. The post-translational modifications of native proteins were analyzed and the anti-aggregation properties of recombinant Emb564, Rab17 and Mgl3 proteins were evaluated in vitro. In addition, the protective effects of the LEA proteins were assessed in living cells under stress in Escherichia coli cells and in Nicotiana bentamiana leaves agroinfiltrated with fluorescent LEA-green fluorescent protein (GFP) fusions. Protein visualization by confocal microscopy indicated that cells expressing Mg3-GFP showed reduced cell shrinkage effects during dehydration and that Rab17-GFP co-localized to leaf oil bodies after heat shock. Overall, the results highlight differences and suggest functional diversity among maize LEA groups.

Mannitol transport and mannitol dehydrogenase activities are coordinated in Olea europaea under salt and osmotic stresses

The intracellular accumulation of organic compatible solutes functioning as osmoprotectants, such as polyols, is an important response mechanism of several plants to drought and salinity. In Olea europaea a mannitol transport system (OeMaT1) was previously characterized as a key player in plant response to salinity. In the present study, heterotrophic sink models, such as olive cell suspensions and fruit tissues, and source leaves were used for analytical, biochemical and molecular studies. The kinetic parameters of mannitol dehydrogenase (MTD) determined in cells growing in mannitol, at 25°C and pH 9.0, were as follows: K(m), 54.5 mM mannitol; and V(max), 0.47 μmol h⁻¹ mg⁻¹ protein. The corresponding cDNA was cloned and named OeMTD1. OeMTD1 expression was correlated with MTD activity, OeMaT1 expression and carrier-mediated mannitol transport in mannitol- and sucrose-grown cells. Furthermore, sucrose-grown cells displayed only residual OeMTD activity, even though high levels of OeMTD1 transcription were observed. There is evidence that OeMTD is regulated at both transcriptional and post-transcriptional levels. MTD activity and OeMTD1 expression were repressed after Na+, K+ and polyethylene glycol (PEG) treatments, in both mannitol- and sucrose-grown cells. In contrast, salt and drought significantly increased mannitol transport activity and OeMaT1 expression. Taken together, these studies support that olive trees cope with salinity and drought by coordinating mannitol transport with intracellular metabolism.

Novel fluorescent conjugate containing glucose and NBD and its carrier-mediated uptake by tobacco cells

Some compounds that contain glucose groups can be transported across the plasma membrane into the cells through hexose transporters. To test the hypothesis that glucose-conjugated insecticides also have similar uptake and translocation properties, a novel fluorescent conjugate (12) was prepared by conjugating glucose and 7-nitrobenz-2-oxa-1,3-diazole with 4-iodo-1-phenylpyrazoles. Its fluorescence spectra and uptake by suspension-cultured tobacco (Nicotiana tabacum L.cv.) cells were studied. The fluorescence spectra showed long wavelengths with maximum emission at 530nm. After incubating tobacco cell suspensions in 10μM conjugate for 0.5h, green fluorescence of 12 was clearly visible in the cells under fluorescence microscopy. After 2h of incubation, more than 70% of 12 was absorbed. Carbonyl cyanide m-chlorophenylhydrazone, phloridzin and glucose drastically inhibited uptake. In concentration-dependent uptakes, the uptake rate of 12 showed a saturable component and was in accordance with Michaelis-Menten kinetics. The results proved that the glucose moiety can guide 12 into tobacco cells and that hexose transporters mediated the uptake.

Purification and functional characterization of protoplasts and intact vacuoles from grape cells

Background

During grape berry ripening, the vacuoles accumulate water, sugars and secondary metabolites, causing great impact in plant productivity and wine quality. However, the molecular basis of these compartmentation processes is still poorly understood. As in many species, the major bottleneck to study these aspects in grapevine is to obtain highly purified vacuoles with a good yield. The present paper describes an isolation method of protoplasts and intact vacuoles from grape berry cells and their functional characterization by transport and cytometric assays.

Findings

Protoplasts were prepared by enzymatic digestion of grape cells, and vacuoles were released and purified by a Ficoll step gradient centrifugation. The tonoplast stained strongly with the fluorescent dye FM1-43 and most vacuoles maintained an internal acidic pH, as assessed by Neutral Red. Flow cytometry analysis of vacuole samples incubated with the calcium-sensitive fluorescent probe Fluo-4 AM revealed a well-defined sub-population of intact vacuoles. As assessed by the pH-sensitive probe ACMA, intact vacuoles generated and maintained a pH gradient through the activity of V-ATPase and V-PPase and were able to transport Ca²⁺ via a proton-dependent transport system.

Conclusions

Highly pure, intact and functional protoplast and vacuole populations from grape cells were obtained with the present method, which revealed to be fast and efficient. The capacity of the vacuole population to sequester protons and accumulate Ca²⁺ strongly suggests the intactness and physiological integrity of these extremely fragile organelles. Grapevine protoplasts and vacuoles may be used as models for both basic research and biotechnological approaches, such as proteomics, solute uptake and compartmentation, toxicological assessments and breeding programs.

Activity of tonoplast proton pumps and Na+/H+ exchange in potato cell cultures is modulated by salt

The efficient exclusion of excess Na from the cytoplasm and vacuolar Na(+) accumulation are the main mechanisms for the adaptation of plants to salt stress. This is typically carried out by transmembrane transport proteins that exclude Na(+) from the cytosol in exchange for H(+), a secondary transport process which is energy-dependent and driven by the proton-motive force generated by plasma-membrane and tonoplast proton pumps. Tonoplast enriched-vesicles from control and 150 mM NaCl-tolerant calli lines were used as a model system to study the activity of V-H(+)-PPase and V-H(+)-ATPase and the involvement of Na(+) compartmentalization into the vacuole as a mechanism of salt tolerance in Solanum tuberosum. Both ATP- and pyrophosphate (PP(i))-dependent H(+)-transport were higher in tonoplast vesicles from the salt-tolerant line than in vesicles from control cells. Western blotting of tonoplast proteins confirmed that changes in V-H(+)-PPase activity are correlated with increased protein amount. Conversely, immunodetection of the A-subunit of V-H(+)-ATPase revealed that a mechanism of post-translational regulation is probably involved. Na(+)-dependent dissipation of a pre-established pH gradient was used to measure Na(+)/H(+) exchange in tonoplast vesicles. The initial rates of proton efflux followed Michaelis-Menten kinetics and the V(max) of proton dissipation was 2-fold higher in NaCl-tolerant calli when compared to the control. H(+)-coupled exchange was specific for Na(+) and Li(+) and not for K(+). The increase of both the pH gradient across the tonoplast and the Na(+)/H(+) antiport activity in response to salt strongly suggests that Na(+) sequestration into the vacuole contributes to salt tolerance in potato.

Phloem transport: cellular pathways and molecular trafficking

The phloem transports nutrients, defensive compounds, and informational signals throughout vascular plants. Sampling the complex components of mobile phloem sap is difficult because of the damage incurred when the pressurized sieve tubes are breached. In this review we discuss sampling methods, the artifacts that can be introduced by different sampling procedures, the intricate pathways by which materials enter and exit the phloem, and the major types of compounds transported. Loading and unloading patterns are largely determined by the conductivity and number of plasmodesmata and the position-dependent function of solute-specific, plasma membrane transport proteins. Recent evidence indicates that mobile proteins and RNA are part of the plant's long-distance communication signaling system. Evidence also exists for the directed transport and sorting of macromolecules as they pass through plasmodesmata. A future challenge is to dissect the molecular and cellular aspects of long-distance macromolecular trafficking in the signal transduction pathways of the whole plant.

Protonophore- and pH-insensitive glucose and sucrose accumulation detected by FRET nanosensors in Arabidopsis root tips

Although soil contains only traces of soluble carbohydrates, plant roots take up glucose and sucrose efficiently when supplied in artificial media. Soluble carbohydrates and other small metabolites found in soil are in part products from exudation from plant roots. The molecular nature of the transporters for uptake and exudation is unknown. Here, fluorescence resonance energy transfer (FRET) glucose and sucrose sensors were used to characterize accumulation and elimination of glucose and sucrose in Arabidopsis roots tips. Using an improved image acquisition set-up, FRET responses to perfusion with carbohydrates were detectable in roots within less than 10 sec and over a wide concentration range. Accumulation was fully reversible within 10-180 sec after glucose or sucrose had been withdrawn; elimination may be caused by metabolism and/or efflux. The rate of elimination was unaffected by pre-incubation with high concentrations of glucose, suggesting that elimination is not due to accumulation in a short-term buffer such as the vacuole. Glucose and sucrose accumulation was insensitive to protonophores, was comparable in media differing in potassium levels, and was similar at pH 5.8, 6.8 and 7.8, suggesting that both influx and efflux may be mediated by proton-independent transport systems. High-resolution expression mapping in root tips showed that only a few proton-dependent transport of the STP (Sugar Transport Protein) and SUT/SUC (Sucrose Transporter/Carrier) families are expressed in the external cell layers of root tips. The root expression maps may help to pinpoint candidate genes for uptake and release of carbohydrates from roots.

Physiological, biochemical and molecular changes occurring during olive development and ripening

Since ancient times the olive tree (Olea europaea), an evergreen drought- and moderately salt-tolerant species, has been cultivated for its oil and fruit in the Mediterranean basin. Olive is unique among the commercial important oil crops for many reasons. Today, it ranks sixth in the world's production of vegetable oils. Due to its nutritional quality, olive oil has a high commercial value compared with most other plant oils. Olive oil has a well-balanced composition of fatty acids, with small amounts of palmitate, and it is highly enriched in the moneonic acid oleate. This makes it both fairly stable against auto-oxidation and suitable for human health. Nevertheless, it is the presence of minor components, in particular phenolics, contributing for oil's high oxidative stability, color and flavor, that makes olive oil unique among other oils. Moreover, as a result of their demonstrated roles in the prevention of cancer and cardiovascular diseases, olive phenolics have gained much attention during the past years. Also unique to virgin olive oil is its characteristic aroma. This results from the formation of volatile compounds, namely, aldehydes and alcohols of six carbon atoms, which is triggered when olives are crushed during the process of oil extraction. The biochemistry of the olive tree is also singular. O. europaea is one of the few species able to synthesize both polyols (mannitol) and oligosaccharides (raffinose and stachyose) as the final products of the photosynthetic CO(2) fixation in the leaf. These carbohydrates, together with sucrose, can be exported from leaves to fruits to fulfill cellular metabolic requirements and act as precursors to oil synthesis. Additionally, developing olives contain active chloroplasts capable of fixing CO(2) and thus contributing to the carbon economy of the fruit. The overall quality of table olives and olive oil is influenced by the fruit ripening stage. Olive fruit ripening is a combination of physiological and biochemical changes influenced by several environmental and cultural conditions, even if most events are under strict genetic control.

Characteristics of glucose uptake by glucose- and NH4-limited grown Penicillium ochrochloron at low, medium and high glucose concentration

Glucose uptake by Penicillium ochrochloron (formerly Penicillium simplicissimum) was studied from 0.01 to 400 mM glucose using chemostat culture and bioreactor batch culture. The characteristics of glucose uptake varied considerably with the conditions of growth, harvest and uptake assay. Glucose-limited grown mycelium showed one saturable transport system [K(S) below 0.01 mM; v(max) 1.1-1.2 mmol (g dry weight)(-1)h(-1)] plus a first order process (permeability P=1.2x10(-7)cm s(-1)). Ammonium-limited grown mycelium showed only one saturable transport system [K(S) 0.3-0.7 mM; v(max) 0.5-0.8 mmol (g dry weight)(-1)h(-1)]. During exponential growth at high glucose concentration (300-400 mM) a first order process was found with a P value of 5.6-9.3x10(-7)cm s(-1). After ammonium exhaustion a second first order phase showed a lower P value (6.1-9.3x10(-8)cm s(-1)). A similar change in permeability was also found after a re-evaluation of published data for Gibberella fujikuroi, Aspergillus niger, Aspergillus awamori and Saccharomycopsis lipolytica. For the first order processes simple diffusion was ruled out as a mechanism for glucose uptake. Glucose uptake by P. ochrochloron was controlled more strongly by metabolism than by transport and was not rate limiting for overflow metabolism.