Dog liver glucose-6-phosphate dehydrogenase: purification and kinetic properties

Glucose-6-phosphate dehydrogenase (G6PD) catalyses the first step of the pentose phosphate pathway which generates NADPH for anabolic pathways and protection systems in liver. G6PD was purified from dog liver with a specific activity of 130 U x mg(-1) and a yield of 18%. PAGE showed two bands on protein staining; only the slower moving band had G6PD activity. The observation of one band on SDS/PAGE with M(r) of 52.5 kDa suggested the faster moving band on native protein staining was the monomeric form of the enzyme.Dog liver G6PD had a pH optimum of 7.8. The activation energy, activation enthalpy, and Q10, for the enzymatic reaction were calculated to be 8.96, 8.34 kcal x mol(-1), and 1.62, respectively.The enzyme obeyed "Rapid Equilibrium Random Bi Bi" kinetic model with Km values of 122 +/- 18 microM for glucose-6-phosphate (G6P) and 10 +/- 1 microM for NADP. G6P and 2-deoxyglucose-6-phosphate were used with catalytic efficiencies (kcat/Km) of 1.86 x 10(6) and 5.55 x 10(6) M(-1) x s(-1), respectively. The intrinsic Km value for 2-deoxyglucose-6-phosphate was 24 +/- 4mM. Deamino-NADP (d-NADP) could replace NADP as coenzyme. With G6P as cosubstrate, Km d-ANADP was 23 +/- 3mM; Km for G6P remained the same as with NADP as coenzyme (122 +/- 18 microM). The catalytic efficiencies of NADP and d-ANADP (G6P as substrate) were 2.28 x 10(7) and 6.76 x 10(6) M(-1) x s(-1), respectively. Dog liver G6PD was inhibited competitively by NADPH (K(i)=12.0 +/- 7.0 microM). Low K(i) indicates tight enzyme:NADPH binding and the importance of NADPH in the regulation of the pentose phosphate pathway.     

Triiodothyronine induces an increase in cyclic GMP in bullfrog tadpole tissues

Cyclic GMP (cGMP) and cyclic AMP (cAMP) were determined in bullfrog tadpole liver and tail fin using 125I-RIA. cGMP increased approx. 100% 1-6 h after the injection of T3 (3 X 10(-10) mol/g body wt.). Reducing the dose of T3 to 1, 3, and 10 X 10(-11) mol/g body wt. provided increases in cGMP of 50-100% above the control value after 2 h. In contrast, only small increases (less than 20%) in cAMP were observed 2-24 h after T3 injection. We conclude that T3 produces a rapid and significant increase in cGMP in the liver and tail fins of premetamorphic tadpoles. These results suggest that thyroid hormones in amphibia may not be an exclusively nucleus-mediated hormone.     

Purification and Characterization of Glucose-6-Phosphate Dehydrogenase from Rat Small Intestine

Glucose-6-phosphate dehydrogenase (G6PD) was purified from rat small intestine with 19.2% yield and had a specific activity of 53.8 units per miligram protein. The pH optimum was determined to be 8.1. The purified rat small intestinal G6PD gave one activity, one protein band on native PAGE. The observation of one band on SDS/PAGE with an Mr of 48 kDa and a specific activity lower than expected may suggest the proteolytically affected enzyme or different form of G6PD in the rat small intestine. The activation energy, activation enthalpy, Q10, and optimum temperature from Arrhenius plot for the rat small intestinal G6PD were found to be 8.52 kcal/mol, 7.90 kcal/mol, 1.59, and 38 degrees C, respectively. The Km values for G6P and NADP+ were 70.1 +/- 20.8 and 23.2 +/- 7.6 microM, respectively. Double-reciprocal plots of 1/Vm versus 1/G6P (at constant [NADP+]) and of 1/Vm versus 1/NADP+ at constant [G6P]) intersected at the same point on the 1/Vm axis to give Vm = 53.8 U/mg protein.     

Binding of thyroxine and triiodothyronine in solubilized nuclear extract of human leukocytes.

An in vitro study was carried out of the interaction of thyroidal hormones with leukocytes and with a solubilized extract of the nuclear fraction of human leukocytes. The respective association constants characterising the binding of triiodothyronine (T3) and thyroxine (T4) to whole leukocytes are roughly equal (for T3, KA = 3.1 X 10(11) 1/mol and for T4, KA = 4 X 10(11) 1/mol) but a 0.4 KCl extract of the nuclear fraction exhibits a different affinity to T3 (KA = 2.16 X 10(11) 1/mol) in comparison with T4 (KA = 1.3 X 10(10) 1/mol). In the nuclear extract, both hormones are bound with the affinity higher for T3 than for T4. The soluble nuclear binding protein in human leukocytes had a molecular weight 46 000, was chromatographically homogenous in chromatography on Sepharose 2B and Sephadex G200, and exhibited a longlasting stability at -25 degrees C, without any marked change in the binding affinity to thyroidal hormones.     

Kinetic properties of human placental glucose-6-phosphate dehydrogenase

The kinetic properties of placental glucose-6-phosphate dehydrogenase were studied, since this enzyme is expected to be an important component of the placental protection system. In this capacity it is also very important for the health of the fetus. The placental enzyme obeyed "Rapid Equilibrium Ordered Bi Bi" sequential kinetics with K(m) values of 40+/-8 microM for glucose-6-phosphate and 20+/-10 microM for NADP. Glucose-6-phosphate, 2-deoxyglucose-6-phosphate and galactose-6-phosphate were used with catalytic efficiencies (k(cat)/K(m)) of 7.4 x 10(6), 4.89 x 10(4) and 1.57 x 10(4) M(-1).s(-1), respectively. The K(m)app values for galactose-6-phosphate and for 2-deoxyglucose-6-phosphate were 10+/-2 and 0.87+/-0.06 mM. With galactose-6-phosphate as substrate, the same K(m) value for NADP as glucose-6-phosphate was obtained and it was independent of galactose-6-phosphate concentration. On the other hand, when 2-deoxyglucose-6-phosphate used as substrate, the K(m) for NADP decreased from 30+/-6 to 10+/-2 microM as the substrate concentration was increased from 0.3 to 1.5 mM. Deamino-NADP, but not NAD, was a coenzyme for placental glucose-6-phosphate dehydrogenase. The catalytic efficiencies of NADP and deamino-NADP (glucose-6-phosphate as substrate) were 1.48 x 10(7) and 4.80 x 10(6) M(-1)s(-1), respectively. With both coenzymes, a hyperbolic saturation and an inhibition above 300 microM coenzyme concentration, was observed. Human placental glucose-6-phosphate dehydrogenase was inhibited competitively by 2,3-diphosphoglycerate (K(i)=15+/-3 mM) and NADPH (K(i)=17.1+/-3.2 microM). The small dissociation constant for the G6PD:NADPH complex pointed to tight enzyme:NADPH binding and the important role of NADPH in the regulation of the pentose phosphate pathway.     

VIP and hormone secretion from thyroidal follicular and C-cells

The effect of vasoactive intestinal peptide (VIP) on the cAMP system of the thyroid and on the secretion of T4 and T3 from the follicular cells and calcitonin and somatostatin from the C-cells was studied in perfused dog thyroid lobes. Activation of the cAMP system was evaluated by measurements of the amount of cAMP released into the perfusion medium. T4, T3, calcitonin and somatostatin were measured by radioimmunoassays. 3 X 10(-6) M VIP induced increases in cAMP release and T4 and T3 secretion from the thyroid while there were no significant alterations in calcitonin and somatostatin release (n = 4). In experiments employing both of the two isolated thyroid lobes 100 microU/ml TSH gave considerably higher increases in T4 and T3 secretion than 10(-6) M VIP (n = 4). The effect of 10(-9) M VIP on T4 and T3 secretion was similar to that of 10(-6) M VIP (n = 4). 10(-10) M VIP induced a small but statistically significant increase in T4 and T3 secretion in two experiments while no effect was observed in two dogs. This high sensitivity of the follicular cells to VIP and the demonstration by others of VIP containing nerves in the thyroid suggest that VIP-ergic nerves may be involved in the regulation of thyroid hormone secretion.     

Glucose-6-phosphate dehydrogenase deficiency in Saudi Arabia. A study in Al-Ula

This paper reports the frequency of glucose-6-phosphate dehydrogenase (G6PD) deficiency in the male and female population of A1-Ula in the northwestern province of Saudi Arabia. The frequency of G6PD deficiency in the male population was 0.098 and in the females it was 0.028. This frequency is significantly lower than those reported for other malaria endemic regions in Arabia. The population was further subgrouped on the basis of their haemoglobin phenotypes and the highest frequency of G6PD deficiency was obtained in male Hb S heterozygotes followed by the male Hb S homozygotes. Phenotyping of G6PD revealed the presence of G6PD-Mediterranean, G6PDA+, G6PDA- and G6PD Mediterranean-like, and the frequency of these variants in Al-Ula was different from those reported in other regions of Saudi Arabia.     

Sublethal ammonia and urea concentrations inhibit rainbow trout (Oncorhynchus mykiss) erythrocyte glucose-6-phosphate dehydrogenase

In vitro and in vivo effects of sublethal ammonia and urea concentrations were assayed on glucose-6-phosphate dehydrogenase (G6PD) of rainbow trout (Oncorhynchus mykiss) erythrocyte. G6PD was purified from erythrocytes with a specific activity of 16.7 EU (mmol NADP+/min)/mg protein and approximately 1600-fold in a yield of approximately 60% by ammonium sulphate precipitation and 2',5'-ADP Sepharose 4B affinity chromatography. The purity of the enzyme was confirmed using SDS polyacrylamide gel electrophoresis. Experiments with ammonia (2.2-5.5 microM) and urea (20-50 microM) showed the inhibitory effects on the enzyme, in vitro. Inhibition effects were determined in vitro by Lineweaver-Burk and regression graphs. The dissociation constant of the enzyme inhibitor complex (Ki) and 50% inhibitory values were 2.26+/-1.21 and 2.86+/-3.51 microM for ammonia and 18.69+/-6.75 and 23.77+/-4.58 microM for urea, respectively. In vivo studies in rainbow trout erythrocytes showed significant (p < 0.01) inhibition of G6PD by ammonia and urea. However, ammonia inhibited more than urea since there were significant differences between the final values of erythrocyte G6PD activities.     

Inactivation of G1ucose-6-Phosphate Dehydrogenase Isozymes from Human and Pig Brain by Aluminum

Prolonged intake of low levels of aluminum from the drinking water has been found to increase the aluminum content in rat brain homogenates and to reduce the activity of hexokinase and glucose-6-phosphate dehydrogenase (G6PD). To determine the interaction of G6PD with aluminum in the brain, we have recently purified two isozymes of G6PD (isozymes I and II) from human and pig brain. Unlike isozyme I, isozyme II also had 6-phosphogluconate dehydrogenase (6-PGD) activity. We report here that G6PD isozymes I and II from human and pig brain purified to apparent homogeneity are inactivated by aluminum. Aluminum did not affect the 6-PGD activity of isozyme II. The aluminum-inactivated enzyme contained 1 mol of aluminum/mol of enzyme subunit. The protein-bound metal ion was not dissociated by exhaustive dialysis at 4 degrees C against 10 mM Tris-HCl (pH 7.0) containing 0.2 mM EDTA. Preincubation of aluminum with citrate, NADP+, EDTA, NaF, ATP, and apotransferrin protected the G6PD isozymes against aluminum inactivation. However, when the G6PD isozymes were completely inactivated by aluminum, only citrate, NaF, and apotransferrin restored the enzyme activity. The dissociation constants for the enzyme-aluminum complex of the isozymes varied from 2 to 4 microM, as measured by using NaF, a known chelator for aluminum. Inhibition of G6PD by low levels of aluminum further strengthens the suggested role of aluminum toxicity in the energy metabolism of the brain.     

Forskolin stimulation of 3, 5, 3'-triiodothyronine release from perifused rat thyroids

In a perifusion system in the presence of 3-isobutyl-1-methylxanthine, forskolin stimulated secretion of not only cAMP but also 3, 5, 3'-triiodothyronine (T3) from rat thyroid glands. The increases in both cAMP and T3 were dose-dependent at forskolin concentrations of 2.0 X 10(-7)M to 2.0 X 10(-5)M. After perifusion for 4 h, tissue concentrations of cAMP also increased as a result of forskolin treatment. Since forskolin is regarded as a specific activator of the cAMP generating system, this observed forskolin stimulation of T3 secretion from perifused rat thyroid glands indicates that cAMP is involved in regulating thyroid hormone secretion.     

High glucose inhibits glucose-6-phosphate dehydrogenase via cAMP in aortic endothelial cells

Recent studies have shown that hyperglycemia is a principal cause of cellular damage in patients with diabetes mellitus. A major consequence of hyperglycemia is increased oxidative stress. Glucose-6-phosphate dehydrogenase (G6PD) plays an essential role in the regulation of oxidative stress by primarily regulating NADPH, the main intracellular reductant. In this paper we show that increased glucose (10-25 mm) caused inhibition of G6PD resulting in decreased NADPH levels in bovine aortic endothelial cells (BAEC). Inhibition was seen within 15 min. High glucose-induced inhibition of G6PD predisposed cells to cell death. High glucose via increased activity of adenylate cyclase also stimulated an increase in cAMP levels in BAEC. Agents that increased cAMP caused a decrease in G6PD activity. Inhibition of cAMP-dependent protein kinase A ameliorated the high glucose-induced inhibition of G6PD. Finally, high glucose stimulated phosphorylation of G6PD. These results suggest that, in BAEC, high glucose stimulated increased cAMP, which led to increased protein kinase A activity, phosphorylation of G6PD, and inhibition of G6PD activity. We conclude that these changes in G6PD activity play an important role in high glucose-induced cell damage/death.     

Acidity and solubility of gramicidin S in water

The acid-base transformations of the gramicidin S molecule in water were studied. The protonization constants of the antibiotic amino group were calculated by the data of the potentiometric titration and the antibiotic distribution in the system of chloroform-water: K1 1.55 X 10(10), K2 1.38 X 10(6), the logarithm of the distribution coefficient of gramicidin S in the system of chloroform-water (1:1) lg alpha G 4.10. By the same data the constants of water solubility of gramicidin S base (1.02 X 10(7) mol/l), gramicidin S monohydrate (1.06 X 10(-4) mol/l) and gramicidin S dihydrochloride (2.08 X 10(-4) mol/l) were calculated.     

Reciprocal effects of epidermal growth factor on key lipogenic enzymes in primary cultures of adult rat hepatocytes. Induction of glucose-6-phosphate dehydrogenase and suppression of malic enzyme and lipogenesis

In primary cultured hepatocytes of adult rats epidermal growth factor (EGF) caused 2- to 3-fold induction of glucose-6-phosphate dehydrogenase (EC 1.1.1.49, G6P dehydrogenase) within 2 days. The effect of EGF was additive with a similar effect of insulin. The half-maximum dose of EGF for the induction was 1 ng/ml. Induction of this enzyme by these hormones was shown by immunotitration to be due to increase of the amount of enzyme. Furthermore, this increase in the amount of enzyme was found to result from increase of syntheses of mRNA and enzyme protein. In contrast, the induction of malic enzyme (EC 1.1.1.40, L-malate:NADP+) oxidoreductase) by insulin plus triiodothyronine was strongly suppressed by the concomitant addition of EGF. Induction of G6P dehydrogenase by EGF, like that by insulin, was not suppressed by either glucagon or dibutyryl cAMP, whereas that of malic enzyme was suppressed additively by EGF and dibutyryl cAMP. EGF also suppressed stimulation of lipogenesis by insulin, measured as incorporation of [1-14C]acetate into triglycerides and phospholipids. Another difference between the inductions of G6P dehydrogenase and malic enzyme was in their dependence on cell density; G6P dehydrogenase induction by insulin and EGF was high at low cell density (3 X 10(4) cells/cm2) and less at higher cell density (13 X 10(4) cells/cm2), whereas induction of malic enzyme was high at higher cell density and less at lower cell density. These results are consistent with the dual role of G6P dehydrogenase in lipogenesis in resting cells and in synthesis of nucleic acid in growing cells. Malic enzyme plays a role only for lipogenesis in mature hepatocytes.     

Red-cell glucose-6-phosphate dehydrogenase of “brown-howler” monkeys (Alouatta fusca)

Physico-chemical properties of erythrocyte glucose-6-phosphate dehydrogenase including erythrocyte G6PD activity, Michaelis constants, K_mG6P and NADP, pH optimum, thermostability and molecular weight were investigated in “brown-howler” monkeys and then compared with the values of human G6PD B(+). The values of Michaelis constants (K_mG6P and NADP) pH optimum were the same as the values of human G6PD B(+). The human G6PD has a dimeric form in the assay conditions employed in the present study, monkey enzyme showing great similariy with human one. Otherwise, the thermostability differed from the human G6PD. The simian enzymatic activity was about four times higher than the human G6PD. A comparison of physico-chemical properties of glucose-6-phosphate dehydrogenase among primates is also presented.     

Transfer of the human X chromosome to human--Chinese hamster cell hybrids via isolated HeLa metaphase chromosomes.

Evidence is presented for the uptake of the human X chromosome by human-Chinese hamster cell hybrids which lack H P R T activity, following incubation with isolated human HeLa S3 chromosomes. Sixteen independent clonal cell lines were isolated in H A T medium, all of which contained a human X chromosome as determined by trypsin-Giemsa staining. The frequency of H A T-resistant clones was 32 x 10(-6) when 10(7) cells were incubated with 10(8) HeLa chromosomes. Potential reversion of the hybrid cells in H A T medium was less than 5 x 10(-7). The 16 isolated cell lines all contained activity of the human X-linked marker enzymes H P R T, P G K,alpha-Gal A, and G6PD, as determined by electrophoresis. The phenotype of G6PD was G6PD A, corresponding to G6PD A in HeLa cells. The human parental cells used in the fusion to form the hybrids had the G6PD B phenotype. The recipient cells gave no evidence of containing human X chromosomes. These results indicate that incorporation and expression of HeLa X chromosomes is accomplished in human-Chinese hamster hybrids which lack a human X chromosome.     

Hormonal interactions with benzodiazepine binding sites in vitro

Prostaglandin A1 and hormones like corticosteroids and DL-Thyroxin (T4) inhibit binding of [3H]RO 5-4864 and [3H] Clonazepam to their respective binding sites with inhibition constants in the low micromolar range. The corticosteroid Cortisone inhibits [3H] RO 5-4864, but not [3H] Clonazepam binding in a competitive manner with an inhibition constant of 4.3 +/- 0.7 microM, Prostaglandin A1 inhibits [3H] Clonazepam, but not [3H] RO 5-4864 binding in a competitive manner with an inhibition constant of 6 +/- 1.2 microM and DL-Thyroxin (T4) inhibits both [3H] RO 5-4864 and [3H] Clonazepam binding with inhibition constants of 12.1 +/- 2.2 and 1.6 +/- 0.4 microM respectively. While the inhibition of [3H] RO 5-4864 binding by DL-Thyroxin (T4) is competitive, the inhibition of [3H] Clonazepam binding is of the mixed type as indicated by Scatchard Plot.     

Chronic effect of TSH on human thyroid tissue in organ culture

The chronic effect of TSH on thyroidal cAMP concentrations and release of thyroid hormones was investigated using human thyroid tissue in organ culture. Normal human thyroid slices were placed in HAM's F-10 synthetic culture medium in Falcon organ tissue culture dishes, and incubated at 37 degrees in a humidified atmosphere of 5% CO2 in air. Medium was changed everyday and daily T3 or T4 release was determined using concentration of T3 or T4 in the medium. After incubation, slices were transferred to the medium containing 10 mM theophylline and incubated without TSH for an additional 30 min to determine thyroidal cAMP concentrations. Thyroidal cAMP concentrations in slices incubated with 10 mU/ml of TSH increased significantly at 2, 6, and 24 hr and even on the 6th day of incubation. Daily T3 release was significantly increased above control from the 3rd day and daily T4 release from the 4th day to the 11th day of incubation with 10 mU/ml of TSH. Histologically, almost all follicles were structurally maintained even on the 11th day of incubation. These results suggest that both thyroidal cAMP concentrations and release of thyroid hormones are stimulated chronically by TSH. This organ culture system is useful for investigating chronic effects of various materials on human thyroid tissue.     

Effects of single-dose external gamma irradiation on rat thyroid status as observed during the year

We studied the rat thyroid status depending on the dose of external radiation and the time passed after the exposure. The experiments were carried out on female albino Wistar rats. The doses absorbed amounted to 0.25; 0.5; 1.0; 2.0 and 5.0 Gy. The animals were decapitated after 3, 6, 24 hours and 7, 30, 180 and 365 days following the radiation. The blood serum was assayed for the contents of thyroxin (T4) and triiodothyronine (T3) using a radioimmunological technique. The liver tissue was assayed spectrophotometrically for the activity of thyroid-induced NADP malate dehydrogenase (NADP-MDH). No changes were found in the blood thyroid hormone contents within short periods after the radiation effect. After 6 hours the T4 levels was 1.2-1.3-fold decreased in the blood of rats receiving the radiation doses of 1.0; 2.0; and 5.0 Gy. After a day the T4 concentration was diminished by 1.21-193-fold in all the experimental animals independently of the radiation dose and that of T3--in 2.0 Gy--and 5.0 Gy--irradiated group. After 7 days following the radiation the T4 and T3 contents remained to be decreased by 1.37-1.75 fold and those of NADP-MDH--by 1.3-1.8-fold in all the animal groups. In a month, the low dose-treated animals (0.25, 0.5, 1.0 Gy) showed the level of thyroid hormones reduced to the control values, whereas the 2.0 and 5.0 Gy--treated rats demonstrated this reduction only by 6 months. The decreased concentration of blood thyroid hormones was due not to the activation of their peripheral metabolism, but, probably, to inhibition of their biosynthesis in thyroid cells under conditions of radiation-induced activation of oxidative stress.     

The inhibitory effects of calmodulin antagonists on TSH-stimulated thyroid hormone release by mouse thyroid lobes

Calcium ions have been shown to play a mojor regulatory role in the release of various hormones from a wide variety of endocrine organs. More recently, in vitro evidence suggests that a calcium-binding protein, calmodulin, is also involved in the release of many hormones. So we examined the effects of several types of calmodulin antagonists on TSH-stimulated thyroid hormone release in vitro. Mouse thyroid lobes (one thyro-tracheal unit/tube) were incubated in Krebs-Ringer bicarbonate buffer at 37 degrees C for 4h. Free thyroxine (fT4) released in the incubation medium, thyroidal cAMP and calmodulin content were measured by RIA. TSH (5 mU/ml) and dibutyryl cAMP (DBC) (200 micrograms/ml) caused a 2-4 fold increase in thyroidal release of fT4. The stimulatory effects of TSH on fT4 release were significantly inhibited by trifluoprazine and prenylamine lactate at the concentration of 5 X 10(-5) M. More specific calmodulin antagonists, W-7 and W-13, were also shown to inhibit TSH stimulation of fT4 release at the concentration of 5 X 10(-5) M. In contrast, TSH stimulation of fT4 release was not depressed by non-specific antagonists, W-5 or W-12, at the same concentration as 5 X 10(-5) M. Further, W-13 also markedly inhibited DBC-stimulated fT4 release. Neither TSH nor PGI2 altered the thyroidal calmodulin content, dissociating with a marked increase in the cAMP concentration. These results suggest that calmodulin plays an important role in TSH-stimulated thyroid hormone release and further that this mechanism exists, at least in part, at the site subsequent to the generation of cAMP.     

Purification and properties of Penicillium glucose 6-phosphate dehydrogenase

1. Glucose 6-phosphate dehydrogenase was isolated and partially purified from a thermophilic fungus, Penicillium duponti, and a mesophilic fungus, Penicillium notatum. 2. The molecular weight of the P. duponti enzyme was found to be 120000+/-10000 by gelfiltration and sucrose-density-gradient-centrifugation techniques. No NADP(+)- or glucose 6-phosphate-induced change in molecular weight could be demonstrated. 3. Glucose 6-phosphate dehydrogenase from the thermophilic fungus was more heat-stable than that from the mesophile. Glucose 6-phosphate, but not NADP(+), protected the enzyme from both the thermophile and the mesophile from thermal inactivation. 4. The K(m) values determined for glucose 6-phosphate dehydrogenase from the thermophile P. duponti were 4.3x10(-5)m-NADP(+) and 1.6x10(-4)m-glucose 6-phosphate; for the enzyme from the mesophile P. notatum the values were 6.2x10(-5)m-NADP(+) and 2.5x10(-4)m-glucose 6-phosphate. 5. Inhibition by NADPH was competitive with respect to both NADP(+) and glucose 6-phosphate for both the P. duponti and P. notatum enzymes. The inhibition pattern indicated a rapid-equilibrium random mechanism, which may or may not involve a dead-end enzyme-NADP(+)-6-phosphogluconolactone complex; however, a compulsory-order mechanism that is consistent with all the results is proposed. 6. The activation energies for the P. duponti and P. notatum glucose 6-phosphate dehydrogenases were 40.2 and 41.4kJ.mol(-1) (9.6 and 9.9kcal.mol(-1)) respectively. 7. Palmitoyl-CoA inhibited P. duponti glucose 6-phosphate dehydrogenase and gave an inhibition constant of 5x10(-6)m. 8. Penicillium glucose 6-phosphate dehydrogenase had a high degree of substrate and coenzyme specificity.