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M J MacDonald

Publications and source records attributed to M J MacDonald.

At least 73 records · Page 4Linked to original sources
PubMed1994

The sequence of a human mitochondrial glycerol-3-phosphate dehydrogenase-encoding cDNA.

A 2618-bp cDNA that encodes the human mitochondrial glycerol-3-phosphate dehydrogenase has been isolated from a HeLa cell cDNA library and the nucleotide sequence determined. An open reading frame encodes a protein of 727 amino acids that is 96% similar to the rat protein and, like the rat protein, contains sites homologous to the Ca(2+)-binding sites of calmodulin, as well as FAD- and putative glycerol-phosphate-binding sites.

Amino Acid Sequence↗
PubMed1994

Sequence of rat mitochondrial glycerol-3-phosphate dehydrogenase cDNA. Evidence for EF-hand calcium-binding domains.

The FAD-dependent, mitochondrial glycerol-3-phosphate dehydrogenase (EC 1.1.99.5) is an essential component of the glycerol phosphate shuttle and is abundant in the pancreatic insulin cell, skeletal muscle, and brain. Although DNA clones for this enzyme and its homologues have been isolated from bacteria and yeast, it has never been cloned from a higher eukaryote. We have cloned and sequenced cDNAs encoding the rat mitochondrial glycerol-3-phosphate dehydrogenase. The longest cDNA (2337 base pairs) encodes a deduced protein of 727 amino acids that shows strong homology to the yeast and bacterial FAD-dependent glycerol phosphate dehydrogenases. The amino terminus of the purified mature protein was sequenced and shows identity with the deduced amino acid sequence beginning with residue 43. The 42 preceding amino acids are consistent with a mitochondrial leader peptide. A highly conserved FAD-binding domain and conserved regions possibly involved with glycerol phosphate binding are present. An unexpected finding was the homology of the deduced protein to calmodulin. Analysis of the deduced protein sequence shows a region near the carboxyl terminus containing two sequences homologous to "EF-hand" calcium-binding domains that are not present in the shorter yeast and bacterial homologues. The second of these domains appears to have features compatible with considerable affinity for calcium, whereas the first does not. The finding of a potential calcium-binding region is consistent with the known enhancement by calcium of the mammalian enzyme activity at low substrate concentrations and the lack of a requirement for calmodulin. This is the first report of EF-hands in a metabolic enzyme or in a mitochondrial protein.

Amino Acid Sequence↗
PubMed1994

Low lactate dehydrogenase and high mitochondrial glycerol phosphate dehydrogenase in pancreatic beta-cells. Potential role in nutrient sensing.

Nutrient metabolism was examined with regard to insulin secretion in purified rat islet beta- and non-beta-cells, beta-cell lines, and hepatocytes. Lactate dehydrogenase (LDH) activity (nanomoles.min-1.mg protein-1) was remarkably low in the glucose-sensitive INS-1 cell line (15.7) and in beta-cells (22.3). Thus, beta-cell LDH was respectively 8-, 122-, 17-, and 136-fold lower than in islet non-beta, liver, HIT-T15, and RINm5F cells. Plasma membrane lactate transport activity was 3-10-fold lower in beta- or INS-1 cells than in the other cell types. Conversely, mitochondrial glycerol phosphate dehydrogenase was strongly expressed only in beta- and INS-1 cells. The significance of these findings to nutrient recognition was explored using INS-1 cells as a model of native beta-cells. Glucose-stimulated lactate output and glucose utilization were, respectively, 12- and 5-fold lower in INS-1 than in RINm5F cells. Each process was entirely blocked by respiratory chain inhibitors in INS-1 cells, whereas glucose utilization was barely affected and lactate output stimulated in RINm5F cells. Glucose oxidation represented 73% of total utilization in INS-1 cells, but only 9% in RINm5F cells. Absolute rates of glucose oxidation, and the extent of mitochondrial NAD(P) reduction, were similar in the two cell types, and glucose stimulated insulin secretion 1.9-fold in INS-1 and 1.4-fold in RINm5F cells. The mitochondrial substrates, monomethyl succinate, pyruvate, and leucine, each triggered secretion in INS-1 cells. The balance of LDH, plasma membrane lactate transport, and mitochondrial glycerol phosphate dehydrogenase activities therefore appear to be important in beta- and INS-1 cell glucose recognition to ensure that mitochondrial oxidation is the principle fate of pyruvate and NADH produced by glycolysis. The resultant close coupling of glycolysis with mitochondrial oxidation explains the absence in beta-cells of Crabtree and Pasteur effects.

Animals↗
PubMed1994

Infrainguinal revascularization in end-stage renal disease.

Patients with end-stage renal disease are being maintained for longer periods with dialysis or renal transplantation. Although renal failure itself is associated with occlusive peripheral vascular disease, such patients often have additional comorbid risk factors. In this series, 88% of patients were diabetic, 93% were hypertensive, and 44% were smokers, all factors that exacerbate the severity of their vasculopathy. As a consequence, the vascular surgeon is increasingly being confronted with limb-threatening peripheral vascular disease in this population. We performed 34 infrainguinal bypasses in 27 patients during a 8-year period from 1986 to 1993. Fifty percent of these were bypasses to the infrapopliteal level. The 12- and 48-month graft patency was 64% and 38%, respectively, by life-table analysis. The limb salvage rate was 65% and 58% at 12 and 48 months. The perioperative mortality rate was 5.9% and the morbidity rate was 37%. Most of the limb loss (66%) occurred during the first 3 months after surgery as a result of acute graft occlusion or nonhealing of an ulcer or minor amputation site. We believe that this reflects an increasingly aggressive approach to limb salvage in patients with end-stage renal disease. Four limbs were lost despite a patent graft. Infrainguinal bypass is a viable management option for limb salvage in patients with end-stage renal disease. These procedures can be undertaken with acceptable perioperative mortality and with a 12-month limb salvage rate of 65%.

Adult↗
PubMed1994

Hemodialysis access in the pediatric patient population.

BACKGROUND: Each year, three to five children per million develop chronic renal failure. Of these, 70% will require dialysis for short periods, and 23% will require prolonged hemodialysis support. It is in the latter group that difficulty is encountered in establishing dialysis access. METHODS: From 1985 to 1992, we provided hemodialysis access for a group of 24 children. There were 16 boys and 8 girls, with a mean age of 11.1 +/- 4 years (range 3 to 17). All children were significantly below the 50th percentile weight for their age and sex. Seven children entered hemodialysis following failed peritoneal dialysis after an average of 21 +/- 10.5 months. Seventeen patients received a renal transplant. Seven of these children have resumed hemodialysis. RESULTS: The technique for establishing hemodialysis was varied: 15 arteriovenous fistulae, 37 expanded polytetrafluoroethylene (ePTFE) bridge grafts, 9 bovine arteriovenous bridge grafts, and 29 chronic central venous catheters. The overall mean functional patency of the fistulae was 6.2 +/- 10.2 months. One third of these fistulae failed to mature sufficiently to permit their use for dialysis purposes. Twenty-one upper extremity ePTFE grafts were implanted, with a mean functional patency of 11 +/- 11.1 months. Sixteen groin loop grafts were utilized, with a primary patency of only 4.1 +/- 5 months. Thrombectomy was performed in 25 cases (patch or interposition in 8 cases), with a secondary patency in these grafts of 10.5 +/- 17 months. An inability to achieve access in 2 children resulted in the creation of unusual types of access: an aorto-caval fistula and an axillo-femoral fistula and a combination of single-needle puncture of an immature fistula with one lumen of a PermCath. There were eight ePTFE graft infections, with graft loss occurring in seven cases. Superior vena caval occlusion occurred in two patients, inferior vena caval thrombosis in one patient, and axillo-subclavian venous occlusion in two patients. Development of central venous occlusions significantly increased the difficulty in establishing dialysis access. The total dialysis period provided by the 90 primary procedures performed in this study was 658 months. Each procedure, therefore, provided access for a mean duration of only 7.3 months. CONCLUSION: Providing dialysis access in the pediatric population is a time-consuming and frustrating challenge. We believe that all patients with renal dysfunction should have their conditions managed as potential long-term dialysis candidates. Therefore, our philosophy is to achieve maximal use from each access site. Although the primary patency of upper-arm ePTFE grafts was greater than that for the forearm fistulae in this study, failure of the upper-arm graft can result in loss of that limb for the purposes of future dialysis access. Consequently, we strongly advocate the "distal before proximal" and "autogenous before prosthetic" dogma in providing pediatric hemodialysis access.

Adolescent↗
PubMed1994

Large amount of (apo)ferritin in the pancreatic insulin cell and its stimulation by glucose.

In screening a rat pancreatic islet cDNA library by differential hybridization for transcripts increased by glucose, the H chain of ferritin, an iron storage protein, was identified. An ELISA for rat ferritin showed that the insulin cell contains a surprisingly high amount of ferritin comparable to that of iron storage tissues. Most of the ferritin was located in the beta cell, as judged from colocalization of ferritin and insulin by immunohistochemical staining of the pancreas. Islets maintained for 24 h in culture medium containing 20 mM glucose are capable of releasing insulin in response to stimulation with glucose, but islets maintained at 1 mM glucose are incapacitated in response to glucose (see ref 1). Immunoassayable ferritin was threefold higher in 20 mM glucose islets than in 1 mM glucose islets and the glucose-stimulated increase in H ferritin mRNA was confirmed by probing Northern blots of islet RNA with authentic H and L chain cDNAs. This showed that H ferritin mRNA was four- to eightfold higher in 20 mM-glucose islets than in 1 mM glucose islets, whereas L ferritin mRNA was decreased 75-90% in 20 mM glucose islets relative to 1 mM glucose islets. The physiological reason for the abundance of (apo)ferritin in the beta cell is not readily apparent because the islet contains very little iron. Whether or not ferritin is used to handle another metal such as zinc, which is plentiful in the beta cell, is unknown. Another potential reason for ferritin in the beta cell is that ferritin has antioxidant properties and the beta cell is particularly sensitive to oxygen radicals. Regardless of its physiologic purpose, the high amount of ferritin can explain why iron is preferentially retained in the insulin cell and causes diabetes in diseases of iron overloading.

Animals↗
PubMed1993

Metabolism of the insulin secretagogue methyl succinate by pancreatic islets.

Previous work demonstrated that methyl esters of succinate are potent insulin secretagogues in pancreatic islets, while unesterified succinate is not. This can be explained by studies reported here, which show that 14C-labeled dimethyl succinate is metabolized to 14CO2 by pancreatic islets, but that 14C-labeled succinic acid is not metabolized. Islets maintained at 1 mM glucose in tissue culture medium for 1 day lose the ability to release insulin in response to glucose and glucose metabolism is decreased 50-80%. The metabolism of dimethyl [1,4-14C]succinate and dimethyl [2,3-14C]succinate is decreased 50-60% in these incapacitated islets relative to islets maintained at 20 mM glucose. From the ratio of 14CO2 formed from dimethyl [1,4-14C]succinate, relative to that from dimethyl [2,3-14C]succinate, "acetate" ratios of 4.9-6.2 were calculated and from the ratio of 14CO2 formed from [2-14C]glucose, relative to that from [6-14C]glucose, "pyruvate ratios" of 1.6-1.7 were calculated. According to the 14CO2 ratios method, these ratios indicate that 53-66% of pyruvate derived from glucose enters the citric acid cycle via carboxylation and 34-47% enters via decarboxylation. Malic enzyme, which carboxylates pyruvate in the cytosol, was normal in islets maintained at 1 mM glucose. Previous work indicated that inhibition of glucose metabolism in islets maintained at low glucose is due to decreased net synthesis of the mitochondrial enzymes pyruvate dehydrogenase and pyruvate carboxylase [J. Biol. Chem. (1991) 266, 22392-22397], which decarboxylate and carboxylate pyruvate, respectively. Acetate (1 mM) but not pyruvate, when added to islets maintained at low glucose, increased dimethyl succinate metabolism to almost that of islets maintained at high glucose. This is consistent with a low amount of pyruvate dehydrogenase being unable to supply acetyl-CoA for condensation with oxalacetate (derived from succinate) and that the rate of the citric acid cycle could be enhanced by adding acetate which can bypass the reaction catalyzed by pyruvate dehydrogenase.

Animals↗
PubMed1993

Estimates of glycolysis, pyruvate (de)carboxylation, pentose phosphate pathway, and methyl succinate metabolism in incapacitated pancreatic islets.

Pancreatic islets were cultured for 24 h in the presence of 1 mM glucose, which renders islets incapable of responding to glucose with insulin release. These islets were compared to islets maintained at 20 mM glucose for 24 h. Detritiation of [2-3H]glucose and [5-3H]glucose in 1 mM glucose islets was normal, suggesting that glucose transport and phosphorylation and all enzymes of glycolysis were not down-regulated in the incapacitated islets. 14CO2 formation from [U-14C]glucose and [6-14C]glucose was inhibited up to 80% and 14CO2 from methyl succinate was inhibited up to 60%, indicating that down-regulation at (a) mitochondrial site(s) might explain the incapacitated insulin release. 14CO2 formation from [3,4-14C]glucose (which becomes [1-14C]pyruvate) was decreased, indicating that the reaction catalyzed by pyruvate dehydrogenase was down-regulated. This decrease, however, was not as large as the decreases in 14CO2 formation from [U-14C]glucose, [2-14C]glucose (which becomes [2-14C]pyruvate), or [6-14C]glucose (which becomes [3-14C]pyruvate), indicating that other reactions were also down-regulated. 14CO2 formation from [1-14C]glucose was inhibited less than that from [6-14C]glucose in the incapacitated islets (34 vs 54%) and these rates indicated that flux of glucose through the pentose phosphate pathway was increased in the incapacitated islet, such that 29% (0.4 nmol of 1.4 glucose/100 islets/90 min) was metabolized via this pathway in the incapacitated islet but only 3.4% (0.1 of 2.9 nmol glucose/100 islets/90 min) was metabolized via the pentose pathway in the 20 mM glucose islets. With rates of 14CO2 evolved from glucose labeled at C2 and C6 and from methyl succinate labeled at C1 + C4 and C2 + C3 the 14CO2 ratio formula was used to calculate the ratios of carboxylated and decarboxylated pyruvate. Roughly equal amounts of pyruvate entered the citric acid cycle by each route in islets maintained for 24 h at 1, 5, or 20 mM glucose. The results indicate that decarboxylation and carboxylation of pyruvate were about equally suppressed in incapacitated islets and that direct inhibition of reactions of the cycle was unlikely. This is consistent with evidence which indicates that down-regulation of both pyruvate carboxylase and pyruvate dehydrogenase occurs in incapacitated islets, i.e., under long-term conditions that modify amounts of enzymes (MacDonald et al., 1991, J. Biol. Chem. 266, 22392-22397).(ABSTRACT TRUNCATED AT 400 WORDS)

Acetates↗
PubMed1993

Glucose enters mitochondrial metabolism via both carboxylation and decarboxylation of pyruvate in pancreatic islets.

The routes by which glucose-derived pyruvate is metabolized to enter mitochondrial pathways to stimulate insulin release in the pancreatic beta cell are unknown. The 14CO2 ratios assay was used to estimate the fractions of glucose-derived pyruvate that enter the citric acid cycle by carboxylation and decarboxylation in pancreatic islets. Pyruvate 14CO2 ratios were estimated with glucose, and acetate 14CO2 ratios were estimated with succinate methyl ester. Glucose and methyl succinate, which are insulin secretagogues, gave more accurate ratios than ratios estimated with pyruvate and acetate, which do not initiate insulin release. The results indicate that relatively equal amounts of pyruvate enter the citric acid cycle by carboxylation and decarboxylation.

Acetates↗
PubMed1992

Lack of glyconeogenesis in pancreatic islets: expression of gluconeogenic enzyme genes in islets.

Studies were performed to obtain evidence for glyconeogenesis from pyruvate to the triose phosphates in pancreatic islets. Inability to show this evidence would be consistent with the fact that glyceraldehyde, but not pyruvate, is a potent insulin secretagogue. Synthesis of 14C-labelled glucose from 14C-labelled pyruvate could not be detected. Since this might have been due to lack of sensitivity required to measure 14C-glucose production in such a scarce tissue as islets, cDNA probes were used to estimate the relative expression of genes coding for gluconeogenic enzymes. Islets expressed pyruvate carboxylase mRNA, but even islets from rats which had been starved (a condition which induces phosphoenolpyruvate carboxykinase (PEPCK) in liver, kidney and adipose tissue) showed no PEPCK mRNA. This is consistent with our previous work showing the absence of PEPCK enzyme activity in islets. Therefore, islets can convert pyruvate to oxalacetate, but since they lack PEPCK, neither the beta nor alpha cell can convert oxalacetate to phosphoenolpyruvate and carry out glyconeogenesis. Pyruvate carboxylase mRNA was increased in islets that possessed the capacity for glucose-induced insulin release versus islets that lacked the capacity to respond to glucose, such as islets from fed rats (versus starved rats) and in islets cultured at a high concentration of glucose (versus at low glucose). Pyruvate carboxylase, therefore, must be involved in pyruvate metabolism and not glyconeogenesis in the pancreatic islet.

Animals↗
PubMed1992

Epitope mapping of human factor VIII inhibitor antibodies by site-directed mutagenesis of a factor VIII polypeptide.

Previous epitope mapping studies of human factor VIII (FVIII) inhibitor antibodies with heavy chain specificity localized epitopes to the amino-terminal half of the FVIII A2 domain. In this report we have used unidirectional deletion analysis and site-directed mutagenesis to identify a minimum length polypeptide and amino acid residues that contribute to the FVIII conformation recognized by these antibodies. Bacterial expression plasmids were exploited to demonstrate that a FVIII polypeptide of approximately 150 residues is required to generate a common heavy chain epitope(s). Another series of plasmids were constructed that synthesize: a FVIII polypeptide containing an internal deletion; four polypeptides with single residue substitutions; two polypeptides with triple residue changes; and a quadruple amino acid replacement within one polypeptide. The relative reactivities of the wild-type and mutant FVIII polypeptides were tested by immunoblotting, inhibitor neutralization assays and ELISA with a variety of human FVIII inhibitor auto- and alloantibodies. These techniques illustrate that the internal deletion mutant and one of the relatively conservative amino acid substitution triple mutants, mutant 389, resulted in significantly decreased immunoreactivity. The data identify FVIII Glu389,390,391 as three critical components of an epitope for human FVIII inhibitor antibodies and identify a major inhibitory epitope involved in the immune response to FVIII.

Amino Acid Sequence↗
PubMed1992

Alternate mechanisms of ras activation are complementary and favor and formation of ras-GTP.

The mechanisms of ras activation by mutations in residue 61 and in the NKXD guanine nucleotide-binding consensus sequence (ras residues 116-119) have been evaluated. Weakly transforming mutations that either reduce intrinsic and GTPase-activating protein (GAP)-stimulated GTPase activities (61P) or enhance guanine nucleotide exchange rates (116H, 119E) were combined into the same H-ras proteins. The resulting double-mutant proteins exhibited significantly stronger transforming forming activities than are observed with each individual mutation, suggesting that the consequences of these two different mechanisms of activation favor maintenance of ras in the active form, which is GTP bound. In vivo nucleotide association analysis demonstrated a direct relationship between ras-GTP formation and transforming activity. Although both 61P and 61L mutations result in reduced intrinsic GTPase activity and loss of GAP stimulation in vitro, only H-ras(61L) exhibits strong transforming activity. While H-ras(61L) is found predominantly in the GTP-bound form, H-ras(61P) is predominantly complexed with GDP in vivo. Thus, in vitro GAP stimulation of GTPase activity does not directly correlate with transforming potential, suggesting that other ras-specific regulatory components may also be important in regulating the cycling of ras between CDP- and GTP-bound states.

3T3 Cells↗
PubMed1991

Pyruvate dehydrogenase and pyruvate carboxylase. Sites of pretranslational regulation by glucose of glucose-induced insulin release in pancreatic islets.

It has been shown previously that glucose-induced insulin release is completely absent in rat pancreatic islets that had been cultured for 1 day at low glucose (1 mM) and that it is restored by culturing islets for a 2nd day at high (20 mM) glucose (MacDonald, M. J., Fahien, L. A., McKenzie, D. I., and Moran, S. M. (1991) Am. J. Physiol. 259, E548-E554). It has been suggested that the incapacitation of glucose's insulinotropism is due to down-regulation of the synthesis of enzymes that process glucose's metabolic signal for insulin release. In the current study, results of metabolic, enzymic, and molecular biologic experiments were each consistent with (an) intramitochondrial site(s) of down-regulation in islets cultured at low glucose. Glucose metabolism was inhibited 80% in islets cultured at 1 mM glucose. The suppression of release of 14CO2 from [6-14C]glucose greater than from [U-14C]glucose greater than [3,4-14C]glucose greater than from [1-14C]glucose in islets cultured at low glucose indicated a mitochondrial site of down-regulation because C-6 of glucose can only be converted to CO2 in the citric acid cycle, whereas C-1 can be released as CO2 in the 6-phosphogluconate dehydrogenase [corrected] reaction, and C-6 of glucose dwells in the citric acid cycle longer than carbons 2-5 of glucose. Since carbons 3 and 4 of glucose can be decarboxylated in the pyruvate dehydrogenase reaction, incomplete suppression of CO2 formation from these carbons is consistent with suppression of pyruvate carboxylation as well as decarboxylation. Formation of 3HOH from [5-3H]glucose was equal in the two groups of islets, indicating that glycolysis as far as phosphoenolpyruvate was intact. This idea was supported by assays which showed that activities of enzymes of the glycolytic pathway between glucokinase/hexokinase and pyruvate kinase were equal in both types of islets. Additional studies indicated that regulation by glucose was at transcription of genes coding for some mitochondrial enzymes. Glucokinase, malic enzyme, and fumarase mRNAs were not affected by glucose, whereas the pyruvate dehydrogenase E1 alpha subunit and pyruvate carboxylase mRNAs were decreased 85-90% in islets cultured at 1 mM glucose. Pyruvate dehydrogenase enzyme activity was decreased to a similar extent in these islets. About 24 h was required for maximal (de)induction of pyruvate dehydrogenase E1 alpha and pyruvate carboxylase mRNAs, and the amounts of transcripts were proportional to the concentrations of glucose between 1 and 20 mM.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Glucose regulates leucine-induced insulin release and the expression of the branched chain ketoacid dehydrogenase E1 alpha subunit gene in pancreatic islets.

Much evidence has accumulated to support the idea that leucine can stimulate insulin release by allosterically activating glutamate dehydrogenase thus enhancing glutamate metabolism. It is less clear how the metabolism of leucine itself contributes to the signal for insulin release. We recently found that culturing pancreatic islets for 1 day at low glucose (1 mM) suppressed glucose-induced insulin release, but preserved leucine-induced insulin release. When islets were cultured at high glucose (20 mM), glucose-induced insulin release was preserved, but leucine-induced insulin release was suppressed (MacDonald, M. J., Fahien, L. A., McKenzie, D. I., and Moran, S. M. (1990) Am. J. Physiol., 259, E548-E554). The suppression of leucine-induced insulin release can be explained by glucose's suppression of the synthesis of the enzyme that catalyzes the first committed step of leucine metabolism, branched chain ketoacid dehydrogenase complex (BCKDH). High glucose suppressed the enzyme activity of the E1 component of the BCKDH complex, as well as the total activity of the BCKDH complex, to usually negligible levels in islets and decreased by an average of 90% the mRNA which encodes E1 alpha, the catalytic subunit of the E1 component of BCKDH, in islets and rat insulinoma cells. Time course studies showed that about 24 h in culture was required to maximally induce or suppress the expression of BCKDH E1 alpha. Culture at high glutamine with or without leucine mimicked to a lesser and more variable degree the effects of high glucose on leucine-induced insulin release and BCKDH E1 alpha mRNA. Leucine-plus-glutamine-induced insulin release was present after culture of islets with glucose and with or without any other secretagogue. Also, glutamate dehydrogenase transcripts and enzyme activity were not significantly altered by varying the concentration of glucose in the culture medium. Thus, leucine's insulinotropism via activation of glutamate dehydrogenase is constitutive. Preproinsulin mRNA levels were markedly increased at high glucose and glyceraldehyde phosphate dehydrogenase transcripts were either unaffected or slightly increased by glucose. Glutamine did not significantly effect the expression of genes other than BCKDH E1 alpha, and leucine had little or no effect on the expression of any of the four genes.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Stimulation of insulin release from pancreatic islets by quinones.

Coenzyme Q (CoQ0) and other quinones were shown to be potent insulin secretagogues in the isolated pancreatic islet. The order of potency was CoQ0 congruent to benzoquinone congruent to hydroquinone-menadione. CoQ6 and CoQ10 (ubiquinone), duroquinone and durohydroquinone did not stimulate insulin release. CoQ0's insulinotropism was enhanced in calcium-free medium and CoQ0 appeared to stimulate only the second phase of insulin release. CoQ0 inhibited inositol mono-, bis- and trisphosphate formation. Inhibitors of mitochondrial respiration (rotenone, antimycin A, FCCP and cyanide) and the calcium channel blocker verapamil, did not inhibit CoQ0-induced insulin release. Dicumarol, an inhibitor of quinone reductase, did not inhibit CoQ0-induced insulin release, but it did inhibit glucose-induced insulin release suggesting that the enzyme and quinones play a role in glucose-induced insulin release. Quinones may stimulate insulin release by mimicking physiologically-occurring quinones, such as CoQ10, by acting on the plasma membrane or in the cytosol. Exogenous quinones may bypass the quinone reductase reaction, as well as many reactions important for exocytosis.

Animals↗