Assay of mannose-6-phosphatase in untreated and detergent-disrupted rat-liver microsomes for assessment of integrity of microsomal preparations

SLC35B1 significantly contributes to the uptake of UDPGA into the endoplasmic reticulum for glucuronidation catalyzed by UDP-glucuronosyltransferases

The transport of UDP-glucuronic acid (UDPGA), a co-substrate of UDP-glucuronosyltransferase (UGT), to the intraluminal side of the endoplasmic reticulum (ER) is an essential step in the glucuronidation of exogenous and endogenous compounds. According to a previous study, the expression of recombinant SLC35B1, SLC35B4, or SLC35D1, nucleotide sugar transporters, in V79 cells has the potential to transport UDPGA into the lumen of microsomes. The purpose of this study is to examine whether the transport of UDPGA by these transporters substantially affects UGT activity. Since the knockdown of UDP-glucose 6-dehydrogenase, a synthetase of UDPGA, in HEK293 cells stably expressing UGT1A1 (HEK/UGT1A1 cells) resulted in a significant decrease in 4-methylumbelliferone (4-MU) glucuronosyltransferase activity, supplementation of a sufficient amount of UDPGA is required for UGT activity. By performing qRT-PCR using cDNA samples from 21 human liver samples, we observed levels of the SLC35B1 and SLC35D1 mRNAs that were 15- and 14-fold higher, respectively, than the levels of the SLC35B4 mRNA, and SLC35B1 showed the largest (37-fold) interindividual variability. Interestingly, 4-MU glucuronosyltransferase activity was significantly decreased upon the knockdown of SLC35B1 in HEK/UGT1A1 cells, and this phenomenon was also observed in HepaRG cells. Using siRNAs targeting 23 different SLC35 subfamilies, the knockdown of SLC35B1 and SLC35E3 decreased 4-MU glucuronosyltransferase activity in HEK/UGT1A1 cells. However, the 4-MU glucuronosyltransferase activity was not altered by SLC35E3 knockdown in HepaRG cells, suggesting that SLC35B1 was the main transporter of UDPGA into the ER in the human liver. In conclusion, SLC35B1 is a key modulator of UGT activity by transporting UDPGA to the intraluminal side of the ER.

Causes of genome instability: the effect of low dose chemical exposures in modern society

Genome instability is a prerequisite for the development of cancer. It occurs when genome maintenance systems fail to safeguard the genome's integrity, whether as a consequence of inherited defects or induced via exposure to environmental agents (chemicals, biological agents and radiation). Thus, genome instability can be defined as an enhanced tendency for the genome to acquire mutations; ranging from changes to the nucleotide sequence to chromosomal gain, rearrangements or loss. This review raises the hypothesis that in addition to known human carcinogens, exposure to low dose of other chemicals present in our modern society could contribute to carcinogenesis by indirectly affecting genome stability. The selected chemicals with their mechanisms of action proposed to indirectly contribute to genome instability are: heavy metals (DNA repair, epigenetic modification, DNA damage signaling, telomere length), acrylamide (DNA repair, chromosome segregation), bisphenol A (epigenetic modification, DNA damage signaling, mitochondrial function, chromosome segregation), benomyl (chromosome segregation), quinones (epigenetic modification) and nano-sized particles (epigenetic pathways, mitochondrial function, chromosome segregation, telomere length). The purpose of this review is to describe the crucial aspects of genome instability, to outline the ways in which environmental chemicals can affect this cancer hallmark and to identify candidate chemicals for further study. The overall aim is to make scientists aware of the increasing need to unravel the underlying mechanisms via which chemicals at low doses can induce genome instability and thus promote carcinogenesis.

Preparation of intact microsomes from cultured mammalian H4IIE cells

INTRODUCTION

Mammalian cell culture is widely used for the cloning and expression of insoluble proteins. The established methods of sub-cellular fractionation of tissues are not always directly suitable for the sub-cellular fractionation of cultured cells. In this study we have optimized the conditions for the preparation of microsomal fractions from cultured cells with the aim of isolating intact vesicles that are suitable for the assay of transport proteins and lumenal enzymes.

METHODS

H4IIE cell cultures were used as a convenient model with high latency of internal endoplasmic reticulum enzyme glucose-6-phosphatase towards mannose-6-phosphate. Also 7-ethoxyresorufin O-deethylase (EROD) activity was determined as a reflection of the state of monooxygenase system.

RESULTS

The variations in a number of homogenization strokes and buffer composition revealed that one homogenization stroke in glass homogenizer with 0.25 M sucrose, 5 mM HEPES, pH 7.4 buffer provides the best latency/activity ratio for homogenates, but for the isolation of microsomes the higher number of strokes (10) as well as low-osmotic buffer (5 mM HEPES, pH 7.4) are needed. However EROD activity is largely reduced in the preparations using buffers containing sucrose, so 5 mM HEPES buffer is recommended as the most suitable to study the microsomal reactions in H4IIE cells.

DISCUSSION

The isolation of microsomes was followed by the significant proteolytic breakdown of the glucose-6-phosphatase enzyme. It is recommended to use cell culture homogenates for assays when possible.

Differential regulation of UDP-GlcUA transport in endoplasmic reticulum and in Golgi membranes

BACKGROUND

In the endoplasmic reticulum (ER), the stimulation of UDP-glucuronosyltransferase (UGT) by UDP-GlcNAc is based on the interaction of transport across the ER membrane of UDP-GlcUA with UDP-GlcNAc. Intramicrosomal UDP-GlcNAc stimulates influx of UDP-GlcUA and thereby enhances delivery of UDP-GlcUA to the catalytic center of UGT in the ER lumen.

AIM

The aim of this study is to investigate whether the interactions between nucleotide sugars for transport across the ER membrane also occur in the Golgi apparatus, and thereby affect UGT activity in Golgi membranes.

RESULTS

We found that Golgi membrane preparations display UGT activity which, unlike in ER membranes, is not stimulated by UDP-GlcNAc. Efflux of intravesicular UDP-GlcNAc and UDP-Xyl marginally enhanced uptake of UDP-GlcUA in Golgi vesicles; such trans-stimulation was much more pronounced in the ER. Efflux of intravesicular UDP-GlcNAc was strongly trans-stimulated by cytosolic UDP-GlcUA in ER-derived vesicles but less so in Golgi-derived vesicles.

CONCLUSION

The interaction between transport of UDP-GlcUA and transport of UDP-GlcNAc or UDP-Xyl is different in Golgi vesicles compared with ER vesicles. This finding is consistent with the different effects of UDP-GlcNAc on glucuronidation in Golgi and ER.

Characterization of the ATP transporter in the reconstituted rough endoplasmic reticulum proteoliposomes

Adenosine triphosphate (ATP) transporter from rat liver rough endoplasmic reticulum (RER) was solubilized and reconstituted into phosphatidylcholine liposomes. The RER proteoliposomes, resulting from optimizing some reconstitution parameters, had an apparent K(m) value of 1.5 microM and a V(max) of 286 pmol min(-1) (mg protein)(-1) and showed higher affinity for ATP and a lower V(max) value than intact RER (K(m) of 6.5 microM and V(max) of 1 nmol). ATP transport was time- and temperature-dependent, inhibited by 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid, which is known as an inhibitor of anion transporters including ATP transporter, but was not affected by atractyloside, a specific inhibitor of mitochondrial ADP/ATP carrier. The internal and external effects of various nucleotides on the ATP transport were examined. ATP transport was cis-inhibited strongly by ADP and weakly by AMP. ADP-preloaded RER proteoliposomes showed a specific increase of ATP transport activity while AMP-preloaded RER proteoliposomes did not show the enhanced overshoot peak in the ATP uptake plot. These results demonstrate the ADP/ATP antiport mechanism of ATP transport in rat liver RER.

Carrier-mediated transport of uridine diphosphoglucuronic acid across the endoplasmic reticulum membrane is a prerequisite for UDP-glucuronosyltransferase activity in rat liver

UDP-glucuronosyltransferases (EC 2.4.1.17) is an isoenzyme family located primarily in the hepatic endoplasmic reticulum (ER) that displays latency of activity both in vitro and in vivo, as assessed respectively in microsomes and in isolated liver. The postulated luminal location of the active site of UDP-glucuronosyltransferases (UGTs) creates a permeability barrier to aglycone and UDP-GlcA access to the enzyme and implies a requirement for the transport of substrates across the ER membrane. The present study shows that the recently demonstrated carrier-mediated transport of UDP-GlcA across the ER membrane is required and rate-limiting for glucuronidation in sealed microsomal vesicles as well as in the intact ER of permeabilized hepatocytes. We found that in both microsomes and permeabilized hepatocytes a gradual inhibition by N-ethylmaleimide (NEM) of UDP-GlcA transport into the ER produced a correspondingly increasing inhibition of 4-methylumbelliferone glucuronidation. That NEM selectively inhibited the UDP-GlcA transporter, without affecting intrinsic UGT activity, was demonstrated by showing that NEM had no effect on glucuronidation in microsomes or hepatocytes with permeabilized ER membrane. Additional evidence that UDP-GlcA transport is rate-limiting for glucuronidation in sealed microsomal vesicles as well as in the intact ER of permeabilized hepatocytes was obtained by showing that gradual selective trans-stimulation of UDP-GlcA transport by UDP-GlcNAc, UDP-Xyl or UDP-Glc in each case produced correspondingly enhanced glucuronidation. Such stimulation of transport and glucuronidation was inhibited completely by NEM, which selectively inhibited UDP-GlcA transport.

Uridine diphosphoxylose enhances hepatic microsomal UDP-glucuronosyltransferase activity by stimulating transport of UDP-glucuronic acid across the endoplasmic reticulum membrane

The UDP-glucuronosyltransferase (UGT) system fulfils a pivotal role in the biotransformation of potentially toxic endogenous and exogenous compounds. Here we report that the activity of UGT in rat liver is stimulated by UDP-xylose. This stimulation was found in native microsomal vesicles as well as in the intact endoplasmic reticulum (ER) membrane, as studied in permeabilized hepatocytes, indicating the potential physiological importance of UDP-xylose in the regulation of UGT. We present evidence that UDP-xylose enhances UGT activity by stimulation of (i) the uptake of UDP-glucuronic acid across the ER membrane and (ii) the elimination of the UDP and/or UMP reaction product out of the ER lumen. UDP-xyloe produced a marked trans-stimulation of microsomal UDP-glucuronic acid uptake when it was present within the lumen of the ER. When UDP-xylose was presented at the cytosolic side of the ER, it acted as a weak inhibitor of UDP-glucuronic acid uptake. Likewise, cytosolic UDP-glucuronic acid strongly trans-stimulated efflux of intravesicular UDP-xylose, whereas cytosolic UDP-xylose was inefficient in trans-stimulating efflux of UDP-glucuronic acid. Microsomal UDP-xylose influx was markedly stimulated by UMP and UDP. Such stimulation was only apparent when microsomes had been preincubated and thereby preloaded with UMP or UDP, indicating that UMP and UDP exeted their effect on UDP-xylose uptake by trans-stimulation from the luminal side of the ER membrane.

Mechanism of stimulation of microsomal UDP-glucuronosyltransferase by UDP-N-acetylglucosamine

We propose the existence in rat liver endoplasmic reticulum (ER) of two asymmetric carrier systems. One system couples UDP-N-acetylglucosamine (UDPGlcNAc) transport to UDP-glucuronic acid (UDPGlcA) transport. When UDPGlcNAc was presented at the cytosolic side of the ER, it then acted as a weak inhibitor of UDPGlcA uptake. By contrast, UDPGlcNAc produced a forceful trans-stimulation of microsomal UDPGlcA uptake when it was present within the lumen of the ER. Likewise, cytosolic UDPGlcA strongly trans-stimulated efflux of intravesicular UDPGlcNAc, whereas cytosolic UDPGlcNAc was ineffective in trans-stimulating efflux of UDPGlcA. A second asymmetric carrier system couples UDPGlcNAc transport to UMP transport. Microsomal UDPGlcNAc influx was markedly stimulated by UMP present inside the microsomes. Such stimulation was only apparent when microsomes had been preincubated and thereby preloaded with UMP, indicating that UMP exerted its effect on UDPGlcNAc uptake by trans-stimulation from the lumenal side of the ER membrane. Contrariwise, extravesicular UMP only minimally trans-stimulated efflux of intramicrosomal UDPGlcNAc. It is widely accepted that UDPGlcNAc acts as a physiological activator of hepatic glucuronidation, but the mechanism of this effect has remained elusive. Based on our findings, we propose a model in which the interaction of two asymmetric transport pathways, i.e. UDPGlcA influx coupled to UDPGlcNAc efflux and UDPGlcNAc influx coupled to UMP efflux, combined with intravesicular metabolism of UDPGlcA, forms a mechanism that leads to stimulation of glucuronidation by UDPGlcNAc.

Carrier-mediated transport of intact UDP-glucuronic acid into the lumen of endoplasmic-reticulum-derived vesicles from rat liver

Uptake and metabolism of UDP-glucuronic acid (UDPGlcA) by rough-endoplasmic-reticulum (RER)-derived vesicles was studied. Analysis of the molecular species, double-labelling experiments and trans-stimulation experiments revealed that initial uptake represented entry into microsomes of predominantly intact UDPGlcA, concomitant with rapid hydrolysis of the internalized nucleotide sugar. The uptake constituted effective translocation from the medium into the lumen of the vesicles. Thus the amount of vesicle-associated label at equilibrium uptake was directly proportional to the volume of the intravesicular space. Permeabilized microsomes were unable to retain UDPGlcA. The microsomal uptake of UDPGlcA met the criteria of bidirectional carrier-mediated translocation. Transport was time- and temperature-dependent, saturable, selective, capable of trans-stimulation, and operational against a concentration gradient. Microsomal uptake was inhibited by N-ethylmaleimide that was presented at the cytosolic side of the endoplasmic-reticulum (ER) membrane. Uptake studies performed in membrane preparations that were highly enriched in RER, smooth ER or Golgi revealed that UDPGlcA was taken up by the ER as well as by the Golgi apparatus. Our findings demonstrate the existence in rat liver ER of a carrier system mediating proper translocation of intact UDPGlcA across the membrane.

Functional characterization of carrier-mediated transport of uridine diphosphate N-acetylglucosamine across the endoplasmic reticulum membrane

Uptake and metabolism of uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) by rough-endoplasmic-reticulum (rER)-derived vesicles was studied. Analysis of the molecular species, double-label experiments, cis-inhibition and trans-stimulation experiments revealed that uptake represented entry of intact UDP-GlcNAc into the microsomal lumen. The amount of vesicle-associated label at equilibrium uptake was directly proportional to the volume of the intravesicular space, and permeabilized microsomes were unable to retain UDP-GlcNAc. These findings indicate that uptake constituted effective translocation from the medium into the lumen of the vesicles. The microsomal uptake of UDP-GlcNAc met the criteria of bidirectional, carrier-mediated translocation. Transport was time and temperature dependent, saturable, selective, capable of trans-stimulation and operational against a concentration gradient. Uptake studies performed in membrane preparations that were highly enriched in either rER, smooth ER, or Golgi revealed that UDP-GlcNAc was taken up by the ER and by the Golgi apparatus.

Topology of nucleotide-sugar:dolichyl phosphate glycosyltransferases involved in the dolichol pathway for protein glycosylation in native rat liver microsomes

Activities of nucleotide-sugar:dolichyl phosphate glycosyltransferases (UDP-N-acetylglucosamine:dolichyl phosphate N-acetylglucosaminyl 1-phosphotransferase, UDP-glucose:dolichyl phosphate glucosyltransferase and GDP-mannose:dolichyl phosphate mannosyltransferase) are not fully expressed in native microsomes and can be enhanced by pretreatment of the microsomes with detergent. To examine whether the latency of dolichyl phosphate glycosyltransferases in native microsomes reflects a lumenal orientation of the catalytic centre, we examined the effect of proteinase treatment of native microsomes on enzymic activity and investigated the relationship between enzymic activity and alteration of the permeability of the microsomal membrane barrier. The enzymic activities catalysing transfer of N-acetylglucosamine and glucose to lipid acceptors were proteinase-sensitive in native sealed microsomes. When various detergents were used to disrupt the membrane barrier, we found no relationship between activity of dolichyl phosphate glycosyltransferases and the latency of mannose-6-phosphatase, which is a marker of the permeability properties of the microsomal membrane. Permeabilization of the endoplasmic reticulum membrane by the pore-forming Staphylococcus aureus alpha-toxin did not affect glycosyltransferase activities. These results do not support the hypothesis that latency of the transferase activities is dependent on the permeability properties of the endoplasmic-reticulum membrane. Collectively our findings can best be explained by postulating that the active centres of the transferases are cytoplasmically oriented, while activation by detergent may be conformation-dependent.

Distinct ryanodine- and inositol 1,4,5-trisphosphate-binding sites in hepatic microsomes

A light hepatic microsomal preparation was fractionated by sucrose-density centrifugation into one rough, one intermediate and two smooth fractions. The four fractions were characterized with respect to parameters relevant to Ca2+ sequestration. Ca2(+)-ATPase activity was similar in the rough, intermediate and smooth I fractions, but lower in the smooth II fraction. Ca2+ accumulation was the highest in the smooth I and intermediate fractions. On the other hand, Ca2+ efflux from the rough fraction was several-fold faster than from the smooth I fraction. All four subfractions exhibited specific binding sites for inositol 1,4,5-trisphosphate (IP3) and ryanodine; however, the receptors were especially enriched in the smooth I fraction. The total binding sites for ryanodine in that fraction exceeded the number of binding sites for IP3 by about 10-fold. The two receptors responded differently to pharmacological agents; caffeine and dantrolene strongly inhibited ryanodine binding but not IP3 binding, whereas heparin inhibited IP3 binding only. Thus the two receptors are distinct entities. The four fractions also showed distinct gel electrophoretic patterns. The use of two different SDS/polyacrylamide-gel gradients and two protein-staining methods revealed major differences in the distribution of the bands corresponding to Mr values of (x 10(-3) 380, 320, 260, 170, 90, 29 and 21. These proteins were enriched in the smooth fraction. The results indicate that the smooth I fraction might have special importance in stimulus-evoked Ca2(+)-release processes.

The effects of Mg2+ on rat liver microsomal Ca2+ sequestration

The effects of Mg2+ on the hepatic microsomal Ca2(+)-sequestering system was tested. Ca2(+)-ATPase activity and Ca2+ uptake were both dependent on the concentration of free Mg2+, reaching maximum levels at 2 mM. The effects of Mg-ATP were also influenced by the concentration of free Mg2+, being maximally effective at a ratio of 1:1. The results suggest that Mg2+ influences Ca2+ sequestration at various steps, namely in addition to forming the substrate of the Ca2(+)-ATPase reaction, Mg-ATP, Mg2+ stimulates the reaction at an additional step, as indicated by its stimulatory effect on the Ca2(+)-ATPase reaction and on Ca2+ uptake, even at optimal Mg-ATP levels. The stimulatory effect of Mg2+ was evident at various pH levels tested, and it was nucleotide specific. The stimulatory effect of Mg2+ might be exerted at the dephosphorylation step of the enzymatic reaction or at an other, yet undefined, site. The results demonstrate a plural effect of Mg2+ on the hepatic microsomal sequestration system. This indicates that, depending on its magnitude, changes in Mg2+ distribution might influence cytosolic Ca2+ levels.

Properties of membrane-bound bilirubin UDP-glucuronyltransferase in rough and smooth endoplasmic reticulum and in the nuclear envelope from rat liver

We examined regulatory properties of bilirubin UDP-glucuronyltransferase in sealed RER (rough endoplasmic reticulum)- and SER (smooth endoplasmic reticulum)-enriched microsomes (microsomal fractions), as well as in nuclear envelope from rat liver. Purity of membrane fractions was verified by electron microscopy and marker studies. Intactness of RER and SER vesicles was ascertained by a high degree of latency of the lumenal marker mannose-6-phosphatase. No major differences in the stimulation of UDP-glucuronyltransferase by detergent or by the presumed physiological activator, UDPGlcNAc, were observed between total microsomes and RER- or SER-enriched microsomes. Isolated nuclear envelopes were present as a partially disrupted membrane system, with approx. 50% loss of mannose-6-phosphatase latency. The nuclear transferase had lost its latency to a similar extent, and the enzyme failed to respond to UDPGlcNAc. Our results underscore the necessity to include data on the integrity of the membrane permeability barrier when reporting regulatory properties of UDP-glucuronyltransferase in different membrane preparations.

Carrier-mediated translocation of uridine diphosphate glucose into the lumen of endoplasmic reticulum-derived vesicles from rat liver

Radiolabeled UDPGlc incubated with rough endoplasmic reticulum (RER)-derived microsomes from rat liver became associated with the vesicles. This microsomal uptake of nucleotide sugar was time and temperature dependent. Analysis of the molecular species containing radiolabel revealed that initial uptake represented entry of predominantly intact UDPGlc in the microsomes. Conclusive evidence for proper translocation of UDPGlc across the microsomal membrane into the intravesicular space was obtained by demonstrating that UDPGlc was transported into an osmotically sensitive compartment. Microsomal uptake of UDPGlc exhibited features characteristic of carrier-mediated transport including saturation, specificity, and countertransport. Inhibition and trans-stimulation studies showed that other uridine-containing nucleotide sugars and 5'-UMP were substrates of the postulated microsomal carrier system for UDPGlc, while cytosine- or guanosine-containing nucleotides and non-5'-uridine monophosphates were, at best, very poor substrates. UDPGlc translocation activities were lower in smooth microsomal fractions than in the RER-derived vesicles, indicating that contamination with Golgi membranes could not be responsible for microsomal transport of UDPGlc. Our findings suggest that rat liver endoplasmic reticulum possesses a carrier system mediating proper translocation of UDPGlc and 5'-uridine-substituted structural analogues across the membrane.

13-cis-retinoic acid stimulates in vitro mannose 6-phosphate hydrolysis and inhibits retinol esterification and benzo[a]pyrene hydroxylation by rat-liver microsomes

13-cis-Retinoic acid, a drug used at high doses in the treatment of recalcitrant acne, increased the permeability of rat-liver microsomal membranes to mannose 6-phosphate in vitro, as indicated by an increase in mannose-6-phosphatase activity. At the same concentrations, four other amphiphiles, including all-trans-retinoic acid, were much less effective. 13-cis-Retinoic acid also inhibited retinol esterification and benzo[a]pyrene hydroxylation in microsome preparations in vitro. Although the molecular mechanism and the reversibility of these effects have not yet been studied, the interaction of 13-cis-retinoic acid with cell membranes may well be involved in both its therapeutic and toxic manifestations.

Topology and regulation of bilirubin UDP-glucuronyltransferase in sealed native microsomes from rat liver

Bilirubin UDP-glucuronyltransferase displays marked latency in native microsomes. To examine whether this latency correlates with structural integrity of the microsomal vesicles and reflects lumenal orientation of the enzyme's catalytic center, we analyzed the relationship between transferase activity and the degree of expression of mannose (Man)-6-phosphatase, which is a marker enzyme of the cisternal face of the ER membrane. Using detergent, sonication, or the pore-forming Staphylococcus aureus alpha-toxin to breach the microsomal membrane permeability barrier, we found that after each of these pretreatments a remarkably close direct relationship existed between latency changes for bilirubin UDP-glucuronyltransferase and Man-6-phosphatase. This finding suggested that the transferase may have the same transverse topology as the phosphohydrolase. We also compared the effects of membrane-impermeant proteinases on bilirubin UDP-glucuronyltransferase activity in native and disrupted microsomes. Whereas the unspecific proteinase nagarse markedly inactivated (to less than 30% of activities in controls) the transferase in disrupted microsomes, treatment with the proteinase had little effect on transferase activity in sealed microsomal vesicles. The results suggest that the active site of bilirubin UDP-glucuronyltransferase is on the lumenal face of the endoplasmic reticulum membrane. It was also found that activation of transferase activity by UDP N-acetylglucosamine, which is the presumed allosteric effector of UDP-glucuronyltransferase, was markedly altered by relatively small changes in structural integrity of the microsomes and totally abolished when latency of Man-6-P hydrolysis fell below approximately 80%. Collectively, these findings demonstrate that the microsomal membrane permeability barrier is a major determinant of expression of microsomal UDP-glucuronyltransferase activity and that quantitative assessment of integrity of the microsomes is essential for studying kinetic properties and regulation of microsomal UDP-glucuronyltransferase.

A new microtechnique for the analysis of the human hepatic microsomal glucose-6-phosphatase system

A microtechnique has been developed which enables a complete kinetic analysis of the human hepatic microsomal glucose-6-phosphatase system to be carried out in microsomes isolated from very small liver samples. Complete or partial deficiencies of any of the proteins of the glucose-6-phosphatase system resulting in Type 1a, 1b, 1c or 1d glycogen storage disease can be therefore be diagnosed using hepatic needle biopsy samples, whereas previous methods of diagnosis needed large wedge biopsy samples requiring laparotomy.

The mechanism of histone activation of the hepatic microsomal glucose-6-phosphatase system: a novel method to assay glucose-6-phosphatase activity

The mechanism of activation of hepatic microsomal glucose-6-phosphatase (EC 3.1.3.9) by histone 2A has been investigated in both intact and disrupted microsomes. Histone 2A increased the Vmax and decreased the Km of glucose-6-phosphatase in intact microsomes but had no effect on glucose-6-phosphatase activity in disrupted microsomes. Histone 2A was shown to activate glucose-6-phosphatase in intact microsomes by disrupting the membrane vesicles and thereby allowing the direct measurement of the activity of the latent glucose-6-phosphatase enzyme. The study demonstrated that disrupting microsomes with histone 2A is an excellent method for directly assaying glucose-6-phosphatase activity as it poses none of the problems encountered with all of the previously used methods.

Glucose-6-phosphate oxidation pathway in rat-liver microsomal vesicles. Stimulation under oxidative stress

The glucose-6-phosphate oxidation pathway present in microsomes was studied using intact microsomal membranes. The oxidation activity, which was measured by monitoring the formation of 14CO2 from [1-14C]glucose 6-phosphate, was greatly stimulated when azodicarboxylic acid bis(dimethylamide), methylene blue or cumene hydroperoxide was added to the assay mixture. Glutathione peroxidase and glutathione reductase are suggested to be involved in the oxidation reaction induced by these oxidizing reagents. We detected a significant activity of the glutathione reductase inherent to microsomes. The microsomal glutathione reductase is latent and requires detergent to reveal its activity. 4,4'-Diisothiocyanostilbene 2,2'-disulfonic acid (DIDS) inhibited the 14CO2 formation, but the inhibition was released by the addition of a detergent. Moreover, the inhibitory effect of DIDS was reversed by glucose 6-phosphate but not by mannose 6-phosphate. We conclude that the glucose-6-phosphate oxidation pathway in intact microsomes starts working under oxidative stress and that a transporter specific for glucose 6-phosphate is involved in the reaction.