Effects of 90 dB pure tone exposure on auditory and cardio-cerebral system functions in macaque monkeys

Excessive noise exposure presents significant health risks to humans, affecting not just the auditory system but also the cardiovascular and central nervous systems. This study focused on three male macaque monkeys as subjects. 90 dB sound pressure level (SPL) pure tone exposure (frequency: 500Hz, repetition rate: 40Hz, 1 min per day, continuously exposed for 5 days) was administered. Assessments were performed before exposure, during exposure, immediately after exposure, and at 7-, 14-, and 28-days post-exposure, employing auditory brainstem response (ABR) tests, electrocardiograms (ECG), and electroencephalograms (EEG). The study found that the average threshold for the Ⅴ wave in the right ear increased by around 30 dB SPL right after exposure (P < 0.01) compared to pre-exposure. This elevation returned to normal within 7 days. The ECG results indicated that one of the macaque monkeys exhibited an RS-type QRS wave, and inverted T waves from immediately after exposure to 14 days, which normalized at 28 days. The other two monkeys showed no significant changes in their ECG parameters. Changes in EEG parameters demonstrated that main brain regions exhibited significant activation at 40Hz during noise exposure. After noise exposure, the power spectral density (PSD) in main brain regions, particularly those represented by the temporal lobe, exhibited a decreasing trend across all frequency bands, with no clear recovery over time. In summary, exposure to 90 dB SPL noise results in impaired auditory systems, aberrant brain functionality, and abnormal electrocardiographic indicators, albeit with individual variations. It has implications for establishing noise protection standards, although the precise mechanisms require further exploration by integrating pathological and behavioral indicators.

Acute exposure of microwave impairs attention process by activating microglial inflammation

BACKGROUND

Attention provides the foundation for cognitions, which was shown to be affected by microwave (MW) radiation. With the ubiquitous of microwaves, public concerns regarding the impact of MW radiation on attention has hence been increased. Our study aims to investigate the potential effect and mechanism of acute microwave exposure on attention.

RESULTS

We identified obvious impairment of attention in mice by the five-choice serial reaction time (5-CSRT) task. Proteomic analysis of the cerebrospinal fluid (CSF) revealed neuroinflammation and microglial activation potentially due to acute MW exposure. Moreover, biochemical analysis further confirmed microglial activation in the prefrontal cortex (PFC) of mice subjected to acute MW exposure. Finally, minocycline, a commercially available anti-inflammatory compound, attenuated neuroinflammation, inhibited the upregulation of N-methyl-D-aspartic acid receptor (NMDAR) including NR2A and NR2B, and also accelerated the attentional recovery after MW exposure.

CONCLUSIONS

We believe that microglial activation and NMDAR upregulation likely contribute to inattention induced by acute MW exposure, and we found that minocycline may be effective in preventing such process.

Update on neurobiological mechanisms of fear: illuminating the direction of mechanism exploration and treatment development of trauma and fear-related disorders

Fear refers to an adaptive response in the face of danger, and the formed fear memory acts as a warning when the individual faces a dangerous situation again, which is of great significance to the survival of humans and animals. Excessive fear response caused by abnormal fear memory can lead to neuropsychiatric disorders. Fear memory has been studied for a long time, which is of a certain guiding effect on the treatment of fear-related disorders. With continuous technological innovations, the study of fear has gradually shifted from the level of brain regions to deeper neural (micro) circuits between brain regions and even within single brain regions, as well as molecular mechanisms. This article briefly outlines the basic knowledge of fear memory and reviews the neurobiological mechanisms of fear extinction and relapse, which aims to provide new insights for future basic research on fear emotions and new ideas for treating trauma and fear-related disorders.

900 MHz electromagnetic field exposure relieved AD-like symptoms on APP/PS1 mice: A potential non-invasive strategy for AD treatment

BACKGROUND

Evidence shows that microwaves radiation may have various biological effects on central nervous system. Role of electromagnetic fields in neurodegenerative diseases, especially AD, has been widely studied, but results of these studies are inconsistent. Therefore, the above effects were verified again and the mechanism was preliminarily discussed.

METHODS

Amyloid precursor protein (APP/PS1) and WT mice were exposed to long-term microwave radiation for 270 days (900 MHz, SAR: 0.25-1.055 W/kg, 2 h/day, alternately), and related indices were assessed at 90, 180 and 270 days. Cognition was evaluated by Morris water maze, Y maze and new object recognition tests. Congo red staining, immunohistochemistry and ELISA were used to analyze Aβ plaques, Aβ40 and Aβ42 content. Differentially expressed proteins in hippocampus between microwave-exposed and unexposed AD mice were identified by proteomics.

RESULTS

Spatial and working memory was improved in AD mice after long-term 900 MHz microwave exposure compared with after sham exposure. Microwave radiation (900 MHz) for 180 or 270 days did not induce Aβ plaque formation in WT mice but inhibited Aβ accumulation in the cerebral cortex and hippocampus in 2- and 5-month-old APP/PS1 mice. This effect mainly occurred in the late stage of the disease and may have been attributed to downregulation of apolipoprotein family member and SNCA expression and excitatory/inhibitory neurotransmitter rebalance in the hippocampus.

CONCLUSIONS

The present results indicated that long-term microwave radiation can retard AD development and exert a beneficial effect against AD, suggesting that 900 MHz microwave exposure may be a potential therapy for AD.

Neural Oscillation Disorder in the Hippocampal CA1 Region of Different Alzheimer's Disease Mice

BACKGROUND

Alzheimer's disease (AD) is a well-known neurodegenerative disease that gradually induces neural network dysfunction and progressive memory deficits. Neural network activity is represented by rhythmic oscillations that influence local field potentials (LFPs). However, changes in hippocampal neural rhythmic oscillations in the early stage of AD remain largely unexplored.

OBJECTIVE

This study investigated neural rhythmic oscillations in 3-month-old APP/PS1 and 5x- FAD mice to assess early neural connectivity in AD.

METHODS

LFPs were recorded from the hippocampal CA1 region with implanted microelectrode arrays while the mice were in the awake resting stage. Welch fast Fourier transforms, continuous wavelet transforms, and phase-amplitude coupling analyses were used to compute the power density of different frequency bands and phase-amplitude modulation indices in the LFPs.

RESULTS

Our results showed impaired theta, low gamma, and high gamma frequency band power in APP/PS1 and 5xFAD mice during the awake resting stage. AD mice also showed decreased delta, alpha, and beta frequency band power. Impaired theta-low gamma and theta-high gamma phaseamplitude coupling were observed in 5xFAD mice.

CONCLUSION

This study revealed neural network activity differences in oscillation power and cross-frequency coupling in the early stage of AD, providing a new perspective for developing biomarkers for early AD diagnosis.

Associations Between a Polymorphism in the Rat 5-HT1A Receptor Gene Promoter Region (rs198585630) and Cognitive Alterations Induced by Microwave Exposure

The nervous system is a sensitive target of electromagnetic radiation (EMR). Chronic microwave exposure can induce cognitive deficits, and 5-HT system is involved in this effect. Genetic polymorphisms lead to individual differences. In this study, we evaluated whether the single-nucleotide polymorphism (SNP) rs198585630 of 5-HT_1A receptor is associated with cognitive alterations in rats after microwave exposure with a frequency of 2.856 GHz and an average power density of 30 mW/cm². Rats were exposed to microwaves for 6 min three times a week for up to 6 weeks. PC12 cells and 293T cells were exposed to microwaves for 5 min up to 3 times at 2 intervals of 5 min. Transcriptional activity of 5-HT_1A receptor promoter containing rs198585630 C/T allele was determined in vitro. Electroencephalograms (EEGs), spatial learning and memory, and mRNA and protein expression of 5-HT_1A receptor were evaluated in vivo. We demonstrated that transcriptional activity of 5-HT_1A receptor promoter containing rs198585630 C allele was higher than that of 5-HT_1A receptor promoter containing T allele. The transcriptional activity of 5-HT_1A receptor promoter was stimulated by 30 mW/cm² microwave exposure, and rs198585630 C allele was more sensitive to microwave exposure, as it showed stronger transcriptional activation. Rats carrying rs198585630 C allele exhibited increased mRNA and protein expression of 5-HT_1A receptor and were more susceptible to 30 mW/cm² microwave exposure, showing cognitive deficits and inhibition of brain electrical activity. These findings suggest SNP rs198585630 of the 5-HT_1A receptor is an important target for further research exploring the mechanisms of hypersensitivity to microwave exposure.

Acute High Level Noise Exposure Can Cause Physiological Dysfunction in Macaque Monkeys: Insight on the Medical Protection for Special Working Environmental Personnel

The high level noise caused by intense acoustic weapons and blasting is a common source of acute acoustic trauma faced by some special environmental personnel. Studies have shown that high level noise can cause auditory and non-auditory effects. However, there are few reports on the biological effects, especially the non-auditory effects of acute high level noise exposure in simulated special working environments, and the great differences between experimental animals and human beings make it difficult to extrapolate from research conclusions. In this study, macaque monkeys were used to detect the effects of acute high level noise exposure on hearing, cognition, and cardiovascular function. Auditory brainstem response, auditory P300, and electrocardiogram (ECG) of macaque monkeys were measured. Results showed that acute high level noise exposure caused permanent hearing threshold shifts; partial hearing loss which couldn’t recover to normal levels in the detection period; pathological changes in T wave and QRS complexes; and large fluctuations in cognitive ability after exposure, which finally recovered to normal. These alterations may be a combination of effects caused by stress-induced neuroendocrine dysfunction and mechanical damage of auditory organs. To elaborate the exact mechanism, further studies are still needed. Meanwhile, positive measures should be taken to reduce the incidence of acute high level noise injury.

Intranasal tetrandrine temperature-sensitive in situ hydrogels for the treatment of microwave-induced brain injury

The brain is the most sensitive organ to microwave radiation. However, few effective drugs are available for the treatment of microwave-induced brain injury due to the poor drug permeation into the brain. Here, intranasal tetrandrine (TET) temperature-sensitive in situ hydrogels (ISGs) were prepared with poloxamers 407 and 188. Its characteristics were evaluated, including rheological properties, drug release in vitro, and mucosal irritation. The pharmacodynamics and brain-targeting effects were also studied. The highly viscous ISGs remained in the nasal cavity for a long time with the sustained release of TET and no obvious ciliary toxicity. Intranasal temperature-sensitive TET ISGs markedly improved the spatial memory and spontaneous exploratory behavior induced by microwave with the Morris water maze (MWM) and the open field test (OFT) compared to the model. The ISGs alleviated the microwave-induced brain damage and inhibited the certain mRNA expressions of calcium channels in the brain. Intranasal temperature-sensitive TET ISGs was rapidly absorbed with a shorter T_max (4.8 h) compared to that of oral TET (8.4 h). The brain targeting index of intranasal temperature-sensitive TET ISGs was as 2.26 times as that of the oral TET. Intranasal temperature-sensitive TET ISGs are a promising brain-targeted medication for the treatment of microwave-induced brain injury.

Radiofrequency radiation at 2.856 GHz does not affect key cellular endpoints in neuron-like PC12 cells

To investigate the potential cytotoxicity of radiofrequency (RF) radiation on central nervous system, rat pheochromocytoma (PC12) cells were exposed to 2.856 GHz RF radiation at a specific absorption rate (SAR) of 4 W/kg for 8 h a day for 2 days in 35 mm Petri dishes. During exposure, the real-time variation of the culture medium temperature was monitored in the first hour. Reactive oxygen species (ROS) production, intracellular Ca²⁺ concentration, and cell apoptosis rate were assessed immediately after exposure by flow cytometry. The results showed that the medium temperature raised about 0.93 °C, but no significant changes were observed in apoptosis, ROS levels or intracellular Ca²⁺ concentration after treatment. Although several studies suggested that RF radiation does indeed cause neurological effects, this study presented inconsistent results, indicating that 2.856 GHz RF radiation exposure at a SAR of 4 W/kg does not have a dramatic impact on PC12 cells, and suggests the need for further investigation on the key cellular endpoints of other nerve cells after exposure to RF radiation.

Non-linear optical imaging and quantitative analysis of the pathological changes in normal and carcinomatous human colorectal muscularis

Non-linear optical (NLO) imaging based on two-photon excitation (2PE) and second harmonic generation (SHG) has been widely used to image microstructures of biomedical specimens over the last two decades. We employed NLO imaging technology to investigate the histology of normal and carcinomatous human colorectal muscularis in transverse and longitudinal views. Results show there are different patterns of pathological changes of muscularis in tissue structure and cell morphology from both views. The NLO imaging provides identical histological information as the H&E images but requires neither stain nor tissue processing. Our study indicates that NLO imaging technology shows more detailed microstructure, which is a critical complementary tool in pathological diagnosis of colorectal tumours. It suggests that NLO imaging could be a very important diagnostic tool to help pathologists realise the real time early detection of human colorectal tumours in the foreseeable future.

Effects of antibiotic resistance genes on the performance and stability of different microbial aggregates in a granular sequencing batch reactor

Antibiotic resistance genes (ARGs) have emerged as key factors in wastewater environmental contaminants and continue to pose a challenge for wastewater treatment processes. With the aim of investigating the performance of granular sludge system when treating wastewater containing a considerable amount of ARGs, a lab-scale granular sequencing batch reactor (GSBR) where flocculent and granular sludge coexisted was designed. The results showed that after inoculation of donor strain NH4(+)-N purification efficiency diminished from 94.7% to 32.8% and recovered to 95.2% after 10 days. Meanwhile, RP4 plasmid had varying effects on different forms of microbial aggregates. As the size of aggregates increased, the abundance of RP4 in sludge decreased. The residence time of RP4 in granules with particle size exceeding 0.9 mm (14 days) was far shorter than that in flocculent sludge (26 days). Therefore, our studies conclude that with increasing number of ARGs being detected in wastewater, the use of granular sludge system in wastewater treatment processes will allow the reduction of ARGs transmissions and lessen potential ecological threats.

Stromal cell-derived factor-1 promotes bone marrow-derived cells differentiation to cardiomyocyte phenotypes in vitro

OBJECTIVE

Recent studies have demonstrated the potential of bone marrow-derived cells (BMDC) to differentiate into cardiomyocytes. Up-regulation of stromal cell-derived factor-1 (SDF-1), a member of the chemokine CXC subfamily, mediating recruitment of BMDC has been documented in infarcted myocardium; however, it remains unknown whether SDF-1 plays a role in cardiomyogenesis of BMDC.

MATERIALS AND METHODS

Adherent BMDCs were cultured with SDF-1, or specific inhibitor for PI3K, CXCR4 or Akt with SDF-1, respectively. After 2 weeks, mRNAs and proteins from BMDCs were examined.

RESULTS

Two weeks after supplementation with SDF-1, either murine or human adherent BMDC cultured in vitro expressed cardiac specific mRNAs (NKX2.5, atrial natriuretic factor and heavy chain beta-myosin) and proteins (troponin I and heavy chain cardiac myosin), and expression levels were partly decreased by combined treatment of CXCR4, PI3K or Akt inhibitor, with SDF-1.

CONCLUSIONS

The novel findings suggest that beyond its role in mobilization and homing of BMDC, SDF-1 can promote BMDC to give rise to cardiomyocyte phenotypes in vitro, and the SDF-1/CXCR4/PI3K/Akt pathway may be one of the molecular mechanisms regulating cardiomyogenesis.

Modeling of branching structures of plants

Previous studies of branching structures generally focused on arteries. Four cost models minimizing total surface area, total volume, total drag and total power losses at a junction point have been proposed to study branching structures. In this paper, we highlight the branching structures of plants and examine which model fits data of branching structures of plants the best. Though the effect of light (e.g. phototropism) and other possible factors are not included in these cost models, a simple cost model with physiological significance, needs to be verified before further research on modeling of branching structures is conducted. Therefore, data are analysed in this paper to determine the best cost model. Branching structures of plants are studied by measuring branching angles and diameters of 234 junctions from four species of plants. The sample includes small junctions, large junctions, two- and three-dimensional junctions, junctions with three branches joining at a point and those with four branches joining at a point. First, junction exponents (x) were determined. Second, log-log plots indicate that model of volume minimization fits data better than other models. Third, one-sided t -tests were used to compare the fitness of four models. It is found that model of volume minimization fits data better than other cost models.

Accuracy in the determination of isoelectric points of some proteins and a peptide by capillary isoelectric focusing: utility of synthetic peptides as isoelectric point markers

To evaluate the accuracy ofisoelectric point determination by capillary isoelectric focusing, the pI values of nine proteins and a peptide, the pI values of which had been determined by other methods and ranging pI 3.55-9.60, were determined by capillary isoelectric focusing by cofocusing of recently developed peptide pI markers ranging 3.38-10.17, and the consistency of the pI values was examined. Isoelectric focusing was carried out in neutral polymer-coated capillaries, and the pH gradient was mobilized by pressure toward the cathode, to detect samples with absorption at 280 nm at a fixed detection point. Carrier ampholytes from two different suppliers and in different pH ranges were used. The sharp peaks of the highly pure peptide pI markers greatly facilitated the unambiguous identification of the peaks. When a carrier ampholyte ranging over the acidic side was used, the detection of acidic pI samples was anomalously delayed. This could be partly mitigated by reducing the viscosity of the anode solution in comparison with the pH gradient formed in the capillary. Since the detection times vs the pH relationships were not linear in most cases, the use of a linear calibration line over an entire pH gradient would be erroneous. Instead, the pI values of samples were calculated by assuming a linear relation for pH against detection time between two flanking marker peptides. Close agreement between the pI values, determined by capillary isoelectric focusing, and the reference values of the samples was observed within an average difference range of 0.04-0.08 pH unit with a sample consumption of 10-100 ng within 30-60 min. Some carrier ampholytes were preferentially more effective at either the acidic or the basic side of the pH gradient. For confirmation of the completion of focusing, the use of two different focusing times is recommended.

In vitro reconstitution of the 24-meric E2 inner core of bovine mitochondrial branched-chain alpha-keto acid dehydrogenase complex: requirement for chaperonins GroEL and GroES

We have investigated the in vitro reconstitution of the 24-meric inner core domain (E2c) of the transacylase (E2) component of bovine branched-chain alpha-keto acid dehydrogenase complex. The yield of recombinant E2c (amino acid residues 161-421 of bovine E2) expressed in Escherichia coli was markedly increased by fusing the bacterial maltose-binding protein (MBP) to the amino terminus of bovine E2c. Following factor Xa digestion to remove the MBP moiety, E2c was completely unfolded in 4.5 M guanidine HCl (Gdn.HCl). The denatured E2c monomers (apparent M(r) = 27,000) were diluted 100-fold at 25 degrees C into a refolding buffer containing 5 mM Mg-ATP and a 4-fold molar excess of chaperonins GroEL and GroES. Full E2 activity was recovered in 45 min. Omission of the chaperonins in the refolding buffer failed to recover any E2 activity. Recovery of E2 activity obeyed hyperbolic kinetics as a function of the chaperonin-to-E2c molar ratio and showed a requirement for hydrolysis of Mg-ATP. A stable GroEL-E2c complex was isolated which, in the presence of GroES and Mg-ATP, generated active E2c 24-mers. Dissociation of recombinant E2c 24-mers into active trimers was achieved by incubation in 1.5 M Gdn.HCl at 25 degrees C. The E2c trimers with an apparent M(r) of 84,000 were isolated by sucrose density gradient centrifugation in the presence of the chaotropic reagent. Removal of 1.5 M Gdn.HCl resulted in the spontaneous reassembly of trimers into the native 24-mer structure independent of chaperonins.(ABSTRACT TRUNCATED AT 250 WORDS)

Renaturation of citrate synthase: influence of denaturant and folding assistants

Citrate synthase (CS), which has been denatured in either guanidine hydrochloride (GdnHCl) or urea can be assisted in its renaturation in a variety of ways. The addition of each of the assistants--bovine serum albumin (BSA), oxaloacetate (OAA), and glycerol--promotes renaturation. In combination, the effect of these substances is additive with respect to the yield of folded CS. The report of Buchner et al. (Buchner, J., Schmidt, M., Fuchs, M., Jaenicke, R., Rudolph, R., Schmid, F.X., & Kiefhaber, T., 1991, Biochemistry 30, 1586-1591) that refolding of CS is facilitated by the GroE system (an Escherichia coli chaperonin [cpn] that is composed of GroEL [cpn60] and GroES [cpn10]) has been confirmed. However, we observed substantially higher yield of reactivated CS, 82%, and almost no reactivation in the absence of GroES, < 5%, whereas Buchner et al. reported 28% and 16%, respectively. In addition, we find that GroE-assisted refolding is more efficient for CS denatured in GdnHCl than for CS denatured in urea. This result is discussed in light of the known difference in the denatured states generated in GdnHCl and urea. Because GroEL inhibits the BSA/glycerol/OAA-assisted refolding, this system will be useful in future studies on the mechanism of GroE-facilitated refolding.

Increased systemic B- and T-lymphocyte responses in hereditary motor and sensory neuropathy (HMSN I)

Immune mechanisms of possible importance for the development and maintenance of peripheral nerve myelin breakdown in HMSN I were analysed by measuring B- and T-cell activation in blood, bone marrow and cerebrospinal fluid. Patients with polyneuropathies of other etiologies served as one control group and patients with tension headache as another. Flow cytometry of blood and bone marrow mononuclear cells revealed that an increased number of CD3+, CD4+ and CD4- CD8- T-cells expressed a late stage activation marker (Ta1). Analysis of T-cells primed for myelin antigens, by studies of IFN-gamma secretion in response to antigen in vitro, showed that both HMSN I and other polyneuropathy patients had low (but significant) numbers of T-cells recognizing whole PNS-myelin. Increased numbers of IgG- and IgM-producing cells were found in blood and bone marrow in the HMSN I patients. Patients with both HMSN I and the other polyneuropathies had few cells in peripheral blood and in bone marrow producing antibodies binding to P2, MAG and MBP in a solid phase immunospot assay. Many cells in the cerebrospinal fluid produced antibodies against MAG. Thus, there was a strong general activation of B- and T-cells in HMSN I while the immunity directed toward peripheral nerve was only slightly elevated. It is an open question if this immune activation is related to the primary gene defect or a secondary event to the nerve damage. The pathogenetic importance of the immune response in maintaining the nerve damage in HMSN I is unclear.

Conformational stability of pig citrate synthase and some active-site mutants

The conformational stabilities of native pig citrate synthase (PCS), a recombinant wild-type PCS, and six active-site mutant pig citrate synthases were studied in thermal denaturation experiments by circular dichroism and in urea denaturation experiments by using DTNB to measure the appearance of latent SH groups. His274 and Asp375 are conserved active-site residues in pig citrate synthase that bind to substrates and are implicated in the catalytic mechanism of the enzyme. By site-directed mutagenesis, His274 was replaced with Gly and Arg, while Asp375 was replaced with Gly, Asn, Glu, or Gln. These modifications were previously shown to result in 10(3)-10(4)-fold reductions in enzyme specific activities. The thermal unfolding of pig citrate synthase and the six mutants in the presence and absence of substrates showed large differences in the thermal stabilities of mutant proteins compared to the wild-type pig citrate synthase. The functions of His274 and Asp375 in ligand binding were measured by oxalacetate protection against urea denaturation. These data indicate that active-site mutations that decrease the specific activity of pig citrate synthase also cause an increase in the conformational stability of the protein. These results suggest that specific electrostatic interactions in the active site of citrate synthase are important in the catalytic mechanism in the chemical transformations as well as the conformational flexibility of the protein, both of which are important for the overall catalytic efficiency of the enzyme.

Immunological mapping of fine molecular surface structures of citrate synthase enzymes from different cell types

Citrate synthase (EC 4.1.3.7), which is present in all living organisms as a key enzyme in aerobic energy metabolism, is one of the most highly phylogenetically conserved enzymes known in terms of its primary and active site structure. However, in terms of other parameters such as in vitro stability, tolerance to changes in pH, degree of self-polymerization, etc., citrate synthases from different sources are markedly different. These divergences can be observed even between isoforms of the enzyme within the same species. Data documenting these diversities suggest that a high degree of difference in tertiary structures may occur. Therefore, the surface profiles of citrate synthase enzymes from yeast, pig, rat, tomato and Escherichia coli were investigated with immunological methods using monoclonal antibody families generated against either pig citrate synthase (alpha-PCS) or yeast citrate synthase-2 (alpha-YCS-2). A high degree of homology of enzyme epitopes was detected on the mitochondrial citrate synthases originating from yeast, tomato, pig and rat cells. Major differences were found between the hexameric citrate synthase originating from E. coli compared with those dimeric forms prepared from eukaryotic cells. Only modest similarities were detected between the highly homologous peroxisomal and mitochondrial yeast citrate synthases. Furthermore, a point mutation of one of the catalytic residues (H274R on recombinant pig and H313R on yeast enzyme) of mitochondrial citrate synthase (CS-1) resulted in a significant increase in immunological similarity with the peroxisomal isoenzyme (CS-2). These findings are discussed in terms of the possible mechanism of evolution of CS-2 in yeast.

Mutation of essential catalytic residues in pig citrate synthase

Two amino acid residues, His274 and Asp375, were replaced singly in the active site of pig citrate synthase (PCS) with Gly274, Arg274, Gly375, Asn375, Glu375, and Gln375. The nonmutant protein and the mutant proteins were expressed in and purified from Escherichia coli, and the effects of these amino acid substitutions on the overall reaction rate and conformation of the PCS protein were studied by initial velocity and full time course kinetic analysis, behavior during affinity column chromatography, and monoclonal antibody reactivity. Native and mutant proteins purified similarly had a subunit molecular weight of 50,000 and were homologous when examined with 10 independent a-PCS monoclonal IgGs or with a polyclonal anti-PHCS serum. No activity was detected for Asn375 or Gln375. The kcats of the other purified mutant proteins, however, were decreased by about 10(3) compared to the nonmutant enzyme activity. The Km for oxalacetate was decreased 10-fold in the Glu375 protein and was reduced by half in Gly274 and Arg274 PCSs, while the Km for acetyl-CoA was decreased 2-3-fold in Gly274, Arg274, and Gln375 PCSs. A mechanism is proposed that electrostatically links His274 and Asp375.