Amide proton transfer and chemical exchange saturation transfer MRI differentiates between growing and non-growing intracranial meningiomas: a pilot study.

AIM: To differentiate between growing and non-growing intracranial meningiomas using magnetisation transfer ratio (MTR) values with amide proton transfer (APT) and chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI). MATERIALS AND METHODS: Seventeen patients with suspected intracranial meningiomas who underwent APT-CEST MRI from November 2020 to April 2021 were evaluated retrospectively. MTR values on APT-CEST imaging as well as conventional MRI features were evaluated. These parameters were compared in growing meningiomas versus non-growing meningiomas and the findings compared with previous MRI examinations. ROC curve analysis was also performed to determine the diagnostic cut-offs for MTR. RESULTS: The cohort comprised 10 patients with growing meningiomas (two men [20%], eight women [80%]; mean age [standard deviation (SD)]: 59.9 years [16]) and seven patients with non-growing meningiomas (seven women [100%]; mean age [SD]: 63.9 years [18.6]). Significant differences were found in MTR values (0.0198 ± 0.0003 versus 0.0131 ± 0.0002; p<0.0001) between the growing meningiomas and non-growing meningiomas groups, respectively. The receiver operating characteristic (ROC) curve analysis showed that MTR values clearly differentiated between growing and non-growing meningiomas. At an area under the ROC curve (AUC) threshold of 0.0151, diagnostic sensitivity, specificity, positive predictive value, and negative predictive values for MTR were 100%, 85.7%, 90.9%, and 100%, respectively. CONCLUSION: Patients with growing meningiomas could be discriminated from patients with non-growing meningiomas, using the MTR values on post-growth tumour APT-CEST imaging.

Muscle atrophy in chronic inflammatory demyelinating polyneuropathy: a computed tomography assessment.

BACKGROUND AND PURPOSE: Muscle atrophy is generally mild in patients with chronic inflammatory demyelinating polyneuropathy (CIDP) compared with the severity and duration of the muscle weakness. Muscle atrophy was evaluated using computed tomography (CT) in patients with CIDP. METHODS: Thirty-one patients with typical CIDP who satisfied the diagnostic criteria for the definite CIDP classification proposed by the European Federation of Neurological Societies and the Peripheral Nerve Society were assessed. The clinicopathological findings in patients with muscle atrophy were also compared with those in patients without atrophy. RESULTS: Computed tomography evidence was found of marked muscle atrophy with findings suggestive of fatty degeneration in 11 of the 31 patients with CIDP. CT-assessed muscle atrophy was in the lower extremities, particularly in the ankle plantarflexor muscles. Muscle weakness, which reflects the presence of muscle atrophy, tended to be more pronounced in the lower extremities than in the upper extremities in patients with muscle atrophy, whereas the upper and lower limbs tended to be equally affected in patients without muscle atrophy. Nerve conduction examinations revealed significantly greater reductions in compound muscle action potential amplitudes in the tibial nerves of patients with muscle atrophy. Sural nerve biopsy findings were similar in both groups. The functional prognoses after immunomodulatory therapies were significantly poorer amongst patients with muscle atrophy. CONCLUSIONS: Muscle atrophy was present in a subgroup of patients with CIDP, including patients with a typical form of the disease. These patients tended to demonstrate predominant motor impairments of the lower extremities and poorer functional prognoses.

Investigation of the thermal tolerance of silicon-based lateral spin valves.

Improvement in the thermal tolerance of Si-based spin devices is realized by employing thermally stable nonmagnetic (NM) electrodes. For Au/Ta/Al electrodes, intermixing between Al atoms and Au atoms occurs at approximately 300 °C, resulting in the formation of a Au/Si interface. The Au-Si liquid phase is formed and diffuses mainly along an in-plane direction between the Si and AlN capping layers, eventually breaking the MgO layer of the ferromagnetic (FM) metal/MgO electrodes, which is located 7 µm away from the NM electrodes. By changing the layer structure of the NM electrode from Au/Ta/Al to Au/Ta, the thermal tolerance is clearly enhanced. Clear spin transport signals are obtained even after annealing at 400 °C. To investigate the effects of Mg insertion in FM electrodes on thermal tolerance, we also compare the thermal tolerance among Fe/Co/MgO, Fe/Co/Mg/MgO and Fe/Co/MgO/Mg contacts. Although a highly efficient spin injection has been reported by insertion of a thin Mg layer below or above the MgO layer, these thermal tolerances decrease obviously.

Current-voltage relations during illumination: photoreceptor membrane of a barnacle.

In voltage clamped photoreceptor cells of the barnacle, light-induced membrane current varied nonlinearly with membrane potential and changed sign at about + 27 millivolts (reversal potential) independently of light intensity. Instantaneous current-voltage relations were linear and intersected the voltage axis at the reversal potential. Illumination increased membrane conductance that was dependent on membrane potential, light intensity, and time.

[Off-pump coronary artery bypass grafting for angina pectoris with coronary artery aneurysm due to kawasaki disease: report of a case].

We report a 27-year-old woman who underwent off-pump coronary artery bypass grafting (OPCAB) for angina pectoris with coronary artery aneurysm due to Kawasaki disease. At the age of 3, she was diagnosed as Kawasaki disease with coronary artery aneurysms in the right coronary artery and the left anterior descending artery. She was medically followed-up since then Because an enlarged aneurysm and a stenotic lesion were recognized in the right coronary artery, operation was indicated. In operation, the right coronary artery was ligated at the inflow and the outflow of the aneurysm. OPCAB was also conducted with the right internal thoracic artery anastomosed to the right coronary artery. Postoperative course was uneventful, and she was discharged at the day 5 after operation. Revascularization using arterial grafts, especially the internal thoracic arteries, may be the choice for young patients to expect a good patency rate in the long-term.

[Off-pump coronary artery bypass grafting as standard operation; surgical strategies and results].

We report the clinical results of 799 cases of isolated coronary artery bypass grafting (CABG) performed during the recent 5 years. We performed off-pump CABG (OPCAB) as standard operation, in which arterial grafts were mainly used. The mean number of distal anastomoses was 3.6 +/- 1.4 per patient Four hundred and fifty-five cases (57.0%) were done only with arterial grafts. Bilateral internal thoracic arteries were used in 326 cases. The mean number of saphenous vein grafts was 1.6 +/- 0.8 per patient. Continuous hemodiafiltraion (CHDF) was performed in 22 cases (2.8%) postoperatively. Among the OPCAB cases, 10 cases (1.3%) were converted to on-pump CABG. There were 7 cases (0.9%) of hospital death. The mean length of postoperative hospital stay was 10.2 +/- 5.3 days. The ratio of the patients with left main trunk disease and that of the patients who required postoperative CHDF increased year by year. The mean length of postoperative hospital stay decreased every year, and the reduced length was 2.7 days in the 5 years (8.7+/- 3.6 days in 2007). It is expected that patients who have severe calcified lesions or who are on hemodialysis may increase in the near future. In such cases, CABG rather than percutaneous catheter intervention may be suitable for revascularization. Therefore, not only appropriate choice of treatment strategies, but also accurate surgical techniques may become more importance.

Neuropathic pain correlates with myelinated fibre loss and cytokine profile in POEMS syndrome.

OBJECTIVE: To reveal characteristic clinicopathological correlates of polyneuropathy, organomegaly, endocrinopathy, monoclonal gammopathy and skin changes (POEMS) syndrome. METHODS: The clinical features of 22 patients with POEMS syndrome were investigated and correlated with the histopathological features of sural nerves and serum cytokine profiles. RESULTS: More than half of the patients complained of pain in the lower extremities, which is closely related to hyperalgesia. Assessment of the total nerve fibre population using complete transverse sural nerve cross-sections, excluding the marked enlargement of endoneurial areas due to intrafascicular oedema, showed that myelinated fibres, especially small myelinated fibres, were reduced, whereas unmyelinated fibres were preserved. Uncompacted myelin lamellae and segmental demyelination were seen more frequently in the small, rather than the large, myelinated fibres. The presence of hyperalgesia was electrophysiologically associated with a reduction of sensory nerve action potentials in the sural nerve (p<0.05) and histopathologically associated with myelinated fibre loss (p<0.01). Serum levels of proinflammatory cytokines (interleukin-1beta, interleukin-6 and tumour necrosis factor-alpha), but not their soluble receptors, were significantly elevated in patients with hyperalgesia (p<0.05-0.01). CONCLUSIONS: Hyperalgesia seen in patients with POEMS syndrome is closely related with a reduction in the myelinated, but not unmyelinated, fibre population. Elevation of proinflammatory cytokines is also correlated with hyperalgesia. The painful symptoms in POEMS syndrome may be generated by well-preserved unmyelinated C-fibres due to the lack of inhibitory myelinated A-fibres, along with cytokine sensitisation.

Prevalence and incidence rates of chronic inflammatory demyelinating polyneuropathy in the Japanese population.

OBJECTIVE AND METHODS: To characterise the epidemiological features of chronic inflammatory demyelinating polyneuropathy (CIDP) in the Japanese population, this study performed a nationwide assessment of the prevalence and incidence rates in Japan. RESULTS: The prevalence rate per 100 000 was 1.61 in the total population; 2.01 in males and 1.23 in females. The age dependent prevalence rates were 0.23 in juveniles (<15 years old), 1.50 in young adults (15-55 years) and 2.31 in elderly adults (>55 years). The sex and age dependent prevalence rates were 0.22 in males and 0.24 in females in juveniles, 1.81 in males and 1.19 in females in young adults, and 3.12 in males and 1.64 in females in elderly adults. The annual incidence rate per 100 000 was 0.48 in the total population, 0.58 in males and 0.38 in females. The age dependent incidence rate was 0.06 in juveniles, 0.40 in young adults and 0.73 in elderly adults. The sex and age dependent incidence rate was 0.05 in males and 0.08 in females in juveniles, 0.50 in males and 0.30 in females in young adults, and 0.93 in males and 0.58 in females in elderly adults. Both the prevalence and incidence rates were very similar throughout the eight geographical areas studied, from the northern to the southern parts of Japan. CONCLUSIONS: The prevalence and incidence rates were similar to those reported in the Caucasian population. The pathogenic background is suggested to be common throughout the different races and geographic areas, while gender and age effects should be taken into account in the pathogenesis of CIDP.

Arterial stiffness after successful renal transplantation.

Cardiovascular disease is a major barrier to the long-term survival of transplant recipients. The aim of this study was to determine whether successful renal transplantation improves the arterial stiffness resulting from chronic renal failure. This study involved a group of 9 recipients (23-56 years) who underwent successful renal transplantation at our clinic. The brachial-ankle pulse wave velocity and--intima-media thickness of the bilateral common carotid arteries were measured in each patient before and 1 year after successful renal transplantation. One year after renal transplantation, the 9 patients showed a mean serum creatinine level of 1.41 mg/dL. Assessment of arterial stiffness in this group revealed that the mean brachial-ankle pulse wave velocity was reduced after renal transplantation, but there was no reduction in the mean intima-media thickness of the bilateral common carotid arteries. There was a significant correlation between the variance ratios of pulse wave velocity and median blood pressure. The more effective blood pressure control provided by renal transplantation may functionally improve arterial stiffness. However, organic arterial stiffness remained unchanged 1 year after transplantation.

Intravenous immunoglobulin therapy markedly ameliorates muscle weakness and severe pain in proximal diabetic neuropathy.

A 57-year-old man with type 2 diabetes mellitus for 10 years showed progressive loss of muscle strength in both legs, pain and muscle atrophy in the femoral region and significant weight loss. On admission, he could not stand alone and used a wheelchair. He also complained of severe pain in the lower extremities. He was diagnosed with proximal diabetic neuropathy (PDN) by characteristic clinical and electrophysiological features. Intravenous immunoglobulin therapy (IVIg 0.4 g/kg x 5 days) markedly reduced the severe pain and muscle weakness in the legs. Eventually, pain assessed by the Visual Analogue Scale was relieved by 80% and muscle strength was also well recovered, thereby enabling the patient to walk with a cane. The present case suggests that IVIg therapy may be effective for the relief of pain in PDN.

Impact of weight gain on the evolution and regression of prediabetes: a quantitative analysis.

BACKGROUND/OBJECTIVES: The quantitative impact of weight gain on prediabetic glucose dysregulation remains unknown; only one study quantitated the impact of weight loss. We quantified the impact of weight gain on the evolution and regression of prediabetes (PDM). SUBJECTS/METHODS: In 4234 subjects without diabetes, using logistic regression analysis with a 4.8-year follow-up period, we analyzed the relationship between (1) δBMI (BMIfollow-up-basal) and the progression from normal glucose regulation (NGR) to PDM or diabetes, and (2) δBMI and the regression from PDM to NGR. RESULTS: Mean (±s.d.) δBMI was 0.17 (±1.3) kg/m2 in subjects with NGR and δBMI was positively and independently related to progression (adjusted odds ratio (ORadj) (95% CI), 1.24 (1.15-1.34), P<0.01). Mean (±s.d.) δBMI was -0.03 (±1.25) kg/m2 in those with PDM and δBMI was negatively related to the regression (ORadj, 0.72 (0.65-0.80), P<0.01). The relation of δBMI to the progression was significant in men (ORadj, 1.42 (1.28-1.59), P<0.01) but not in women (ORadj, 1.05 (0.94-1.19), P=0.36). Also, the negative impact of δBMI on the regression was significant only in men (men, ORadj, 0.65 (0.57-0.75), P<0.01; women, ORadj, 0.94 (0.77-1.14), P=0.51). CONCLUSIONS: In Japanese adults, an increase in the BMI by even 1 kg/m2 was related to 24% increase in the risk of development of PDM or diabetes in NGR subjects and was related to 28% reduction in the regression from PDM to NGR. In women, we did not note any significant impact of weight gain on the evolution or regression of PDM.

Predictive models for conversion of prediabetes to diabetes.

AIM: To clarify the natural course of prediabetes and develop predictive models for conversion to diabetes. METHODS: A retrospective longitudinal study of 2105 adults with prediabetes was carried out with a mean observation period of 4.7years. Models were developed using multivariate logistic regression analysis and verified by 10-fold cross-validation. The relationship between [final BMI minus baseline BMI] (δBMI) and incident diabetes was analyzed post hoc by comparing the diabetes conversion rate for low (< -0.31kg/m2) and high δBMI (≥ -0.31kg/m2) subjects after matching the two groups for the covariates. RESULTS: Diabetes developed in 252 (2.5%/year), and positive family history, male sex, higher systolic blood pressure, plasma glucose (fasting and 1h- and 2h-values during 75g OGTT), hemoglobin A1c (HbA1c) and alanine aminotransferase were significant, independent predictors for the conversion. By using a risk score (RS) that took account of all these variables, incident diabetes was predicted with an area under the ROC curve (95% CI) of 0.80 (0.70-0.87) and a specificity of prediction of 61.8% at 80% sensitivity. On division of the participants into high- (n=248), intermediate- (n=336) and low-risk (n=1521) populations, the conversion rates were 40.1%, 18.5% and 5.9%, respectively. The conversion rate was lower in subjects with low than high δBMI (9.2% vs 14.4%, p=0.003). CONCLUSIONS: Prediabetes conversion to diabetes could be predicted with accuracy, and weight reduction during the observation was associated with lowered conversion rate.

Intravenous immunoglobulin treatment in painful sensory neuropathy without sensory ataxia associated with Sjögren's syndrome.

Patients having neuropathy associated with Sjögren's syndrome may present with pain and superficial sensory involvement in the absence of sensory ataxia. Treatment for this form of associated neuropathy has not been established. The case of a patient with painful sensory neuropathy associated with Sjögren's syndrome, whose symptoms, particularly pain, responded well to intravenous immunoglobulin both at onset and in a relapse, is reported. Other patients with painful sensory neuropathy associated with Sjögren's syndrome may also be candidates for intravenous Ig treatment.

Prevention of hypoxia-induced pulmonary hypertension by enhancement of endogenous heme oxygenase-1 in the rat.

We investigated the role of heme oxygenase (HO)-1 in the development of hypoxia-induced pulmonary hypertension. HO catalyzes the breakdown of heme to the antioxidant bilirubin and the vasodilator carbon monoxide. Hypoxia is a potent but transient inducer of HO-1 in vascular smooth muscle cells in vitro and in the lung in vivo. By using agonists of HO-1, we sustained a high expression of HO-1 in the lungs of rats for 1 week. We report that this in vivo enhancement of HO-1 in the lung prevented the development of hypoxic pulmonary hypertension and inhibited the structural remodeling of the pulmonary vessels. The mechanism(s) underlying this effect may involve a direct vasodilating and antiproliferative action of endogenous carbon monoxide, as well as an indirect effect of carbon monoxide on the production of vasoconstrictors. These results provide evidence that enhancement of endogenous adaptive responses may be used to prevent hypoxia-induced pulmonary hypertension.

Clinicopathologic features of nonsystemic vasculitic neuropathy and microscopic polyangiitis-associated neuropathy: a comparative study.

OBJECTIVE: To compare clinicopathologic findings in nonsystemic vasculitic neuropathy (NSVN) and microscopic polyangiitis-associated neuropathy (MPAN). METHODS: Patients clinicopathologically confirmed to have NSVN (n=23) or MPAN (n=40) were compared with respect to clinical, electrophysiologic, and histopathologic features. RESULTS: Clinical features of neuropathy such as initial symptoms, progression, and distribution of sensory and motor deficits were similar in both groups, while functional compromise was greater in MPAN than NSVN. Abnormalities of laboratory data including those reflecting severity and extent of inflammation such as C-reactive protein were more conspicuous in MPAN than NSVN. Perinuclear anti-neutrophil cytoplasmic antibodies (p-ANCA) were positive in two-thirds of patients with MPAN but negative in all NSVN. Electrophysiologic and histopathologic findings indicated axonal neuropathy in both groups, whereas the reduction of compound muscle action potentials in the tibial nerve and sensory nerve action potentials in the median nerve was significantly more profound in MPAN than NSVN. As for the epineurial perivascular infiltration, frequencies of cell-specific markers for T lymphocytes, macrophages, and B lymphocytes among cells infiltrating the vasculitic lesions were essentially similar between groups. CONCLUSIONS: Clinicopathologic profiles and vascular pathology were similar between NSVN and MPAN but the age at onset, severity, and presence of p-ANCA were clearly different. Further study is needed to clarify the pathogenesis of NSVN and its place in the vasculitic spectrum of diseases.

Why is inorganic phosphate necessary for uncoupling of oxidative phosphorylation by Cd2+ in rat liver mitochondria?

The phosphate (Pi)-dependent uncoupling action of Cd2+ in oxidative phosphorylation in rat liver mitochondria was studied mainly in terms of Pi transport. Cd2+ at 2 microM caused full uncoupling in the presence of 10 mM Pi, but no uncoupling in the absence of Pi. Cd2+ released state 4 respiration after a certain lag-time, and then the respiration increased progressively with time. After its addition, Cd2+ was taken up by mitochondria in a similar period to the lag time before respiratory release. KIH-201, a potent and specific inhibitor of Pi transport via the Pi/H+ symporter, abolished the uncoupling completely. Cd2+ caused dissipation of the electric transmembrane potential (delta psi) and swelling of mitochondria in a Pi-dependent manner. Uncoupling by Cd2+ was found to take place in parallel with the uptake of Pi into mitochondria via the Pi/H+ symporter, suggesting that the uncoupling was due to acceleration of H+ influx through the Pi/H+ symporter activated by Cd2+.

Protective effect of the plasticizer di(2-ethylhexyl) phthalate against damage of the mitochondrial membrane induced by calcium: possible participation of the adenine nucleotide translocator.

The effect of di(2-ethylhexyl) phthalate (DEHP) on the response of isolated rat liver mitochondria to Ca2+ was investigated. DEHP was found to inhibit more than 60% of the auto-accelerating release of respiration induced by 100 microM Ca2+, being maximally inhibitory at 40 microM. Prior addition of DEHP also partially inhibited Ca2+-induced swelling of the mitochondrial matrix. However, DEHP did not change the net rate of Ca2+ uptake measured by the steady-state infusion method. DEHP also reduced the rate of adenine nucleotide exchange across the mitochondrial membrane. Another alkyl phthalate and alkyl citrates had similar effects on Ca2+-induced membrane damage, but their potencies depended on the lengths of their alkyl chains. These results suggest that the effects of DEHP and other alkyl esters on mitochondrial functions are mainly based on their actions on membrane lipids surrounding adenine nucleotide translocator (AdNT), resulting in alteration of the interaction between these phospholipids and AdNT, and consequent modification of the state of the protein.

Thermoluminescence and flash-induced oxygen yield in herbicide resistant mutants of the D1 protein in Synechococcus PCC7942.

Several strains of Synechococcus PCC7942 carrying point mutations in the gene psbA were studied by thermoluminescence and polarographic measurement of flash-induced oxygen yield. The following results were obtained: (a) Replacement of Ser-264 in D1 by Ala (mutant Di1) or Gly (mutant G264) resulting in DCMU and atrazine resistance leads to a downshift of the thermoluminescence (TL) B-band peak temperature from 40 degrees C in wild-type thylakoids to about 30 degrees C. In dark adapted samples of both mutants the TL and oxygen yield pattern induced by a train of single turnover flashes were strongly damped indicative of a high miss factor. (b) In contrast to Ser-264 mutants, replacement of Phe-255 in D1 by Tyr (mutant Tyr5) induced strong resistance to atrazine but not to DCMU and did not affect the peak termperature of the B-band and the flash-induced TL and oxygen yield patterns. In this respect mutant Tyr5 resembles the wild type. (c) No significant differences have been found between strains with single site mutations in psbAI and normal psbAII/psbAIII genes, and strains with same mutations in psbAI but additional deletion of psbAII and psbAIII. Obviously in strains were psbAI is present, PS II complexes containing gene products of psbAII and psbAIII are not assembled in detectable amounts. (d) Strains with double mutations at positions 264 and 255 display a downshift of the B-band peak temperature. Their oscillatory patterns of B-band intensity and oxygen yield are highly damped. This behaviour is similar to strains D1 and G264 which are modified at position 264 only. We extend reports on additivity of mutation effects on herbicide binding to binding of QB. (e) Mutations at the QB site not only influence the binding of QB and herbicides but also change the thermoluminescence quantum yield and the lifetimes of the redox states S2 and S3 of the water oxidase. This finding might indicate long ranging effects on Photosystem II exerted by structural modifications of the QB site. From these data we conclude that Ser-264 is essential for binding of atrazine, DCMU and QB, whereas Phe-255 is involved in atrazine binding and its substitution by Tyr does not markedly affect QB or DCMU binding in Synechococcus PCC7942.

Hyperpolarization of a barnacle photoreceptor membrane following illumination.

Membrane potential changes following illumination of a photoreceptor cell in the lateral ocellus of a barnacle (Balanus eburneus) were studied by means of intracellular recording and polarization techniques. Illumination produces a depolarizing response. When the illumination is terminated, the membrane potential temporarily becomes more negative than the resting potential prior to illumination. Although the amplitude of this postillumination hyperpolarization depends upon the intensity as well as the duration of the light pulse, the time course is fairly constant. The hyperpolarization is not associated with any significant membrane conductance increase and is abolished by 10(-5)M ouabain. It diminishes when the external Na or K ions are removed. An intracellular injection of Na ions produces a hyperpolarization similar to that following illumination. It is suggested that the postillumination hyperpolarization is produced by an electrogenic Na pump which is activated by the Na influx during illumination.

1.8 A crystal structure of the major NAD(P)H:FMN oxidoreductase of a bioluminescent bacterium, Vibrio fischeri: overall structure, cofactor and substrate-analog binding, and comparison with related flavoproteins.

We have solved the crystal structure of FRase I, the major NAD(P)H:FMN oxidoreductase of Vibrio fischeri, by the multiple isomorphous replacement method (MIR) at 1.8 A resolution with the conventional R factor of 0.187. The crystal structure of FRase I complexed with its competitive inhibitor, dicoumarol, has also been solved at 2.2 A resolution with the conventional R factor of 0.161. FRase I is a homodimer, having one FMN cofactor per subunit, which is situated at the interface of two subunits. The overall fold can be divided into two domains; 80% of the residues form a rigid core and the remaining, a small flexible domain. The overall core folding is similar to those of an NADPH-dependent flavin reductase of Vibrio harveyi (FRP) and the NADH oxidase of Thermus thermophilus (NOX) in spite of the very low identity in amino acid sequences (10% with FRP and 21% with NOX). 56% of alpha-carbons of FRase I core residues could be superposed onto NOX counterparts with an r.m.s. distance of 1.2 A. The remaining residues have relatively high B-values and may be essential for defining the substrate specificity. Indeed, one of them, Phe124, was found to participate in the binding of dicoumarol through stacking to one of the rings of dicoumarol. Upon binding of dicoumarol, most of the exposed re-face of the FMN cofactor is buried, which is consistent with the ping pong bi bi catalytic mechanism.