The regulation of plant lignin biosynthesis under boron deficiency conditions.

Boron (B) is a required micronutrient that is crucial for the growth and development of vascular plants. A deficiency in B is generally regarded as a limiting factor affecting agricultural production in many parts of the world. Boron is involved in the metabolism of plant lignin and additionally, B deficiency can lead to the excessive accumulation of lignin in plant leaves/roots, resulting in corking symptoms and inhibited growth. However, the effect of B on lignin biosynthesis is not as well characterized as the specific function of B in the cell wall. In this article, recent studies on the regulation of lignin biosynthesis in plants under low-B stress conditions are reviewed. Moreover, the following possible mechanisms underlying the lignin synthesis promoted by B deficiency are discussed: (1) the accumulation of phenolic substances during B deficiency directly enhances lignin synthesis; (2) excess H2 O2 has a dual function to the enhancement of lignin under boron deficiency conditions, serving as a substrate and a signaling molecule; and (3) B deficiency regulates lignin synthesis through the expression of genes encoding transcription factors such as MYBs. Finally, future studies regarding physiology, molecules, and transcriptional regulation may reveal the mechanism(s) mediating the relationship between lignin synthesis and B deficiency. This review provides new insights and important references for future research and the enhancement of plant B nutrition.

The Xyloglucan Endotransglucosylase/Hydrolase Gene XTH22/TCH4 Regulates Plant Growth by Disrupting the Cell Wall Homeostasis in Arabidopsis under Boron Deficiency.

TCH4 is a xyloglucan endotransglucosylase/hydrolase (XTH) family member. Extensive studies have shown that XTHs are very important in cell wall homeostasis for plant growth and development. Boron (B), as an essential micronutrient for plants, plays an essential role in the cross-linking of cell wall pectin. However, the effect of B on cell wall organization is unclear. This study aimed to explore the mechanism of plant adaption to B stress by investigating the role of TCH4 in cell wall homeostasis. We conducted both plate and hydroponic cultures of wild-type Col-0 and overexpression and gene knockout lines of XTH22/TCH4 to analyze the phenotype, components, and characteristics of the cell wall using immunofluorescence, atomic force microscopy (AFM), and transmission electron microscopy (TEM). B deficiency induces the expression of TCH4. The overexpression lines of TCH4 presented more sensitivity to B deficiency than the wild-type Col-0, while the knockout lines of TCH4 were more resistant to low B stress. Up-regulation of TCH4 influenced the ratio of chelator-soluble pectin to alkali-soluble pectin and decreased the degree of methylesterification of pectin under B-deficient conditions. Moreover, we found that B deficiency disturbed the arrangement of cellulose, enlarged the gap between cellulose microfibrils, and decreased the mechanical strength of the cell wall, leading to the formation of a thickened and deformed triangular region of the cell wall. These symptoms were more profound in the TCH4 overexpression lines. Consistently, compared with Col-0, the O2- and MDA contents in the TCH4 overexpression lines increased under B-deficient conditions. This study identified the B-deficiency-induced TCH4 gene, which regulates cell wall homeostasis to influence plant growth under B-deficient conditions.

Boron phenyl alanine targeted chitosan-PNIPAAm core-shell thermo-responsive nanoparticles: boosting drug delivery to glioblastoma in BNCT.

Boron neutron capture therapy (BNCT) is one of the best treatment modalities for glioblastoma multiform that could selectively kill the tumor cells. To be successful in BNCT, it is crucial to have enough 10B in the tumor. l-boron phenylalanine (l-BPA) targeted thermo-responsive core-shell nanoparticles (NPs) of chitosan-poly(N-isopropylacrylamide) (PNIPAAm) were our idea for endocytosis via sialic acid receptors, and selective delivery of 10B to glial cells. Methotrexate (MTX) was chosen as a model drug for evaluating the efficacy of NPs in tumor cells, and BPA was selected for BNCT purposes. The polymeric conjugates were synthesized and the chemical structures were approved by spectroscopic methods (FTIR, 1H NMR, and 11B NMR). Cargos were loaded efficiently (>95%) in the prepared NPs, and the release profile of MTX and BPA was studied around the lower critical solution temperature (LCST; about 39 °C). The loaded drugs were released quantitatively at the LCST, while almost no drug was released at 37 °C. The prepared NPs did not show considerable hemolysis ratio (<2%) and were still safe when loaded BPA, on U87MG cells. The MTX loaded NPs showed lower IC50 (30.78 µg/mL) than the free MTX (37.03 µg/mL) in MTT assay, and targeted NPs had the lowest IC50s in U87MG cell lines (27.35 µg/mL). Targeted BPA@CSSU-PNI NPs were uptaken better than the non-targeted ones by U87MG cells, and CR-39 assay showed the boron content efficiency for further applications in BNCT. This study's results introduce novel targeted thermo-responsive NPs for treating glioblastoma using BNCT.

Boron toxicity in higher plants: an update.

MAIN CONCLUSION: In this review, emphasis is given to the most recent updates about morpho-anatomical, physiological, biochemical and molecular responses adopted by plants to cope with B excess. Boron (B) is a unique micronutrient for plants given that the range of B concentration from its essentiality to toxicity is extremely narrow, and also because it occurs as an uncharged molecule (boric acid) which can pass lipid bilayers without any degree of controls, as occurs for other ionic nutrients. Boron frequently exceeds the plant's requirement in arid and semiarid environments due to poor drainage, and in agricultural soils close to coastal areas due to the intrusion of B-rich seawater in fresh aquifer or because of dispersion of seawater aerosol. Global releases of elemental B through weathering, volcanic and geothermal processes are also relevant in enriching B concentration in some areas. Considerable progress has been made in understanding how plants react to B toxicity and relevant efforts have been made to investigate: (I) B uptake and in planta partitioning, (II) physiological, biochemical, and molecular changes induced by B excess, with particular emphasis to the effects on the photosynthetic process, the B-triggered oxidative stress and responses of the antioxidant apparatus to B toxicity, and finally (III) mechanisms of B tolerance. Recent findings addressing the effects of B toxicity are reviewed here, intending to clarify the effect of B excess and to propose new perspectives aimed at driving future researches on the topic.

Boron removal by a composite sorbent: Polyethylenimine/tannic acid derivative immobilized in alginate hydrogel beads.

A novel composite material was prepared by the grafting of tannic acid on polyethylenimine (PEI), which allows an efficient sorption of boron (sorption capacity close to 0.89 mmol B g-1). The encapsulation of this chelating sorbent (finely crushed) facilitates its use (readily solid/liquid separation, use in fixed-bed columns) at the expense of a loss in sorption capacity (proportionally decreased by the introduction of alginate having poor efficiency for boron uptake). Sorption isotherms are modeled using the Langmuir equation, while the kinetic profiles are presented a good fit by pseudo-second order rate equation. In addition, the encapsulating matrix introduces supplementary resistance to intraparticle diffusion, especially when the resin is dried without control: freeze-drying partially limits this effect. The stability (at long-term storage) of the sorbent is improved when the sorbent is stored under nitrogen atmosphere. The presence of an excess of NaCl was investigated. The degradation of the hydrogel (by ion-exchange of Ca(II) with Na(I)) leads to a decrease in the sorption performance of composite material but the action of Ca(II) ions in the solutions re-stabilizes the hydrogel.

"Exogenous boron alleviates salt stress in cotton by maintaining cell wall structure and ion homeostasis".

Salt stress is considered one of the major abiotic stresses that impair agricultural production, while boron (B) is indispensable for plant cell composition and has also been found to alleviate salt stress. However, the regulatory mechanism of how B improves salt resistance via cell wall modification remains unknown. The present study primarily focused on investigating the mechanisms of B-mediated alleviation of salt stress in terms of osmotic substances, cell wall structure and components and ion homeostasis. The results showed that salt stress hindered plant biomass and root growth in cotton. Moreover, salt stress disrupted the morphology of the root cell wall as evidenced by Transmission Electron Microscope (TEM) analysis. The presence of B effectively alleviated these adverse effects, promoting the accumulation of proline, soluble protein, and soluble sugar, while reducing the content of Na+ and Cl- and augmenting the content of K+ and Ca2+ in the roots. Furthermore, X-ray diffraction (XRD) analysis demonstrated a decline in the crystallinity of roots cellulose. Boron supply also reduced the contents of chelated pectin and alkali-soluble pectin. Fourier-transform infrared spectroscopy (FTIR) analysis further affirmed that exogenous B led to a decline in cellulose accumulation. In conclusion, B offered a promising strategy for mitigating the adverse impact of salt stress and enhancing plant growth by countering osmotic and ionic stresses and modifying root cell wall components. This study may provide invaluable insights into the role of B in ameliorating the effects of salt stress on plants, which could have implications for sustainable agriculture.

Boron-mediated lignin metabolism in response to aluminum toxicity in citrus (Poncirus trifoliata (L.) Raf.) root.

Aluminum (Al) toxicity has conspicuous detrimental effects on citrus production whereas boron (B) has been shown to alleviate its toxicity. Lignin plays a critical role in the cell wall extensibility and root elongation under stressed conditions. Hence, the interaction between B and Al on cell wall structure and lignin-related metabolic pathway was investigated in root of trifoliate orange (Poncirus trifoliata (L.) Raf.) seedlings. The results showed B supply considerably decreased the Al content in root, particularly in cell wall, and reduced Al-induced damage on growth-related parameters and thickness of cell wall. Boron application decreased the hydrogen peroxide (H2O2), malondialdehyde (MDA), and lignin contents in the Al-treated root, which prevents the inhibitory effects of Al on the root length. Moreover, metabonomics results showed that B addition resulted in the reduction of metabolites involved in the lignin biosynthesis pathways (phenylpropanoid metabolic) i.e., shikimic acid, tyrosine, caffeic acid, chlorogenic acid, coniferyl alcohol, sinapinic acid, sinapaldehyde, and sinapyl alcohol, as well as distinctively restrain the activities of lignin biosynthesis-related enzymes (4-coumarate-CoA ligase (4CL), cinnamyl-alcohol dehydrogenase (CAD)) under Al toxicity. Collectively, our findings suggest that the positive effects of B on the resistance of Al toxicity may be it reduces Al accumulation in the cell wall, lignin biosynthesis, and cell wall thickness, thereby increasing the extensibility and elasticity of cell wall and thus promoting root elongation.

Chitosan Coating Effect on Cellular Uptake of PLGA Nanoparticles for Boron Neutron Capture Therapy.

The usefulness of poly(lactide-co-glycolide) nanoparticles as a boron compound carrier for boron neutron capture therapy has been recently reported. In this study, chitosan-modified poly(DL-lactide-co-glycolide) (PLGA) nanoparticles were prepared to better facilitate the delivery of boron to the tumor. Chitosan hydroxypropyltrimonium chloride (CS), which can easily be modified for compatibility with PLGA nanoparticles, was used as chitosan. o-Carborane-loaded PLGA nanoparticles (bare nanoparticles) with a mean volume diameter of 111.4 ± 30.1 nm, and o-Carborane-loaded PLGA nanoparticles coated with CS (CS-coated nanoparticles) with a mean volume diameter of 113.6 ± 32.5 nm were prepared via an emulsion solvent evaporation method. Electrophoretic mobility was measured to calculate the particle surface charge number density of these particles; particle surface charge number densities of -1.91 mM and 20.8 mM were obtained for the bare and CS-coated nanoparticles, respectively. This result indicates that the particle surface was fully covered with CS. In vitro cellular uptake tests were carried out by using B16 melanoma cells. From the results of observation via confocal laser scanning microscopy, it was revealed that CS-coated nanoparticles existed around the cell nucleus, and were localized in the cytoplasm. Cellular uptakes of bare and CS-coated nanoparticles were quantitatively assessed by using fluorescence-activated cell sorting; the mean fluorescence intensity of CS-coated nanoparticles was three times higher than that of bare nanoparticles. The number of boron atoms in B16 melanoma cells was also investigated. Inductively coupled plasma atomic emission spectroscopy revealed that the number of boron atoms per cell of CS-coated nanoparticles was 1.8 times higher than that of bare nanoparticles. Based on these findings, we consider CS-coated nanoparticles to be suitable for boron neutron capture therapy.

Label-free DNA sensor by boron-doped diamond electrode using an ac impedimetric approach.

An electrochemical biosensor using a boron-doped diamond (BDD) electrode is described for differentiating between gene sequences according to DNA hybridization events using an ac impedimetric approach. BDD electrodes were dipped into a 1% solution of polyethylenimine (PEI) to adsorb a thin layer of positively charged PEI on the surface of BDD, then PEI-modified BDD electrodes were used to immobilize negatively charged single-stranded PCR fragments from Exon 7 of human p53 gene. Alternating current impedimetric measurements were first performed on these systems in phosphate buffered saline (PBS) and then upon exposure to single-stranded DNA (ssDNA). When the ssDNA-immobilized BDD electrode and solution ssDNA were completely complementary, a large drop in impedance was measured. Complementary DNA could be clearly detected at concentrations down to 10 (-19) g mL (-1) at a fixed frequency (10 Hz). Higher concentrations of DNA gave faster hybridization with saturation occurring at levels above 1.0 pg mL (-1.) Responses were much lower upon exposure to noncDNA, even at higher concentrations. The results show it is possible to directly detect target DNA at a fixed frequency and without additional labeling.

Mobility of boron-polyol complexes in the hemiparasitic association between Rhinanthus minor and Hordeum vulgare: the effects of nitrogen nutrition.

Boron (B) is an essential nutrient required for plant growth and physiological processes. Long-distance B transport is facilitated by the formation of B-polyol complexes. We investigated B uptake and distribution in response to differing levels of exogenous nitrogen supply in the hemiparasitic association between Rhinanthus minor and Hordeum vulgare (barley) and in unparasitised barley and single Rhinanthus plants. In this system, the polyol mannitol is the major assimilate in Rhinanthus, whereas polyols are not detectable in barley. Furthermore, previous studies have shown that the accumulation of polyols within Rhinanthus is negatively affected by the application of exogenous nitrogen. Within the association, the strongest accumulation of B was detected in lateral buds and inflorescences of Rhinanthus, consistent with the greatest B demand in strong sink organs supplied through the phloem that contain high concentrations of mannitol. In the host, the strongest B accumulation was found in xylem-supported leaf lamellae. Roots and sheaths did not accumulate substantial amounts of B, while re-circulation of B through the phloem vessels accounted for only 10% (unparasitised) and 8% (parasitised) of the xylem sap-imported B in the mannitol-free barley hosts. In contrast, 53% (attached) and 39% (in the absence of a host) of the xylem sap-imported B was re-circulated in the phloem in the mannitol-rich Rhinanthus. We therefore present the first quantitative uptake and flow models of long-distance B transport in polyol-rich and polyol-free plants. Our findings are consistent with a close relationship between B re-translocation and mannitol concentrations in phloem vessels.

Ameliorative effects of boron on aluminum induced variations of cell wall cellulose and pectin components in trifoliate orange (Poncirus trifoliate (L.) Raf.) rootstock.

Aluminum (Al) phytotoxicity is a major limitation in the production of crops in the soils with pH ≤ 5. Boron (B) is indispensable nutrient for the development of higher plants and B role has been reported in the alleviation Al toxicity. Trifoliate orange rootstock was grown in two B and two Al concentrations. The results of the present study showed that Al toxicity adversely inhibited root elongation and exhibited higher oxidative stress in terms of H2O2 and O2- under B-deficiency. Additionally, the X-ray diffraction (XRD) analysis confirmed the increase of the cellulose crystallinity in the cell wall (CW). Al-induced remarkable variations in the CW components were prominent in terms of alkali-soluble pectin, 2-keto-3-deoxyoctonic acid (KDO) and the degree of methyl-esterification (DME) of pectin. Interesting, B supply reduced the pectin (alkali-soluble) under Al toxicity. Moreover, the results of FTIR (Fourier transform infrared spectroscopy) and 13C-NMR (13C nuclear magnetic resonance) spectra revealed the decrease of carboxyl groups and cellulose by B application during Al exposure. Furthermore, B supply tended to decrease the Al uptake, CW thickness and callose formation. The study concluded that B could mitigate Al phytotoxicity by shielding potential Al binding sites and by reducing Al induced alterations in the CW cellulose and pectin components.

Study of the boron levels in serum after implantation of different ratios nano-hexagonal boron nitride-hydroxy apatite in rat femurs.

Boron and its derivatives are effective in bone recovery and osteointegration. However, increasing the boron levels in body liquids may cause toxicity. The aim of our study is to investigate serum boron levels using ICP-MS after implantation of different ratios of nano-hBN-HA composites in rat femurs. All rats were (n=126) divided into five experimental groups (n=24) and one healthy group (6 rats); healthy (Group1), femoral defect + %100 HA (Group2), femoral defect + %2.5 hBN + %97.5 HA (Group3), femoral defect + %5 hBN + %95 HA (Group4), femoral defect + %10 hBN + %90 HA (Group5), femoral defect + %100 hBN (Group6). The femoral defect was created in the distal femur (3mm drill-bit). Each implant group was divided into four different groups (n=24) also 6 rats sacrificed for each groups in one week intervals during four weeks. In our results; at 1, 2, 3, and 4 weeks after implantation near bone tissue, serum levels of boron were evaluated using ICP-MS. We demonstrated that neither short-term nor long-term implantation of hBN-HA composite resulted in statistically increased serum boron levels in experimental groups compared to healthy group. In conclusion, this study investigated the implant material produced form hBN-HA for the first time. Our data suggest that hBN is a new promising target for biomaterial and implant bioengineers.

Qualitative autoradiography with polycarbonate foils enables histological and track analyses on the same section.

Neutron autoradiography is an imaging methodology that enables analysis of the spatial distribution of heavy ion emitters in a given material. In particular, it allows localization of (10)B in a tissue section put in contact with a nuclear track detector. Boron imaging is essential when considering boron neutron capture therapy as an option for treating cancerous tumors. A description of the autoradiography method is presented together with specific characteristics and technical details developed in our laboratory. We propose a new mounting technique to compare autoradiography images with the same section that gave rise to the latent tracks. The solid state nuclear track detector is polycarbonate, because it can be processed rapidly to obtain the autoradiographic results. It is a transparent material, which allows visualization of the sections mounted on it. Tissue can be removed easily and background is minimal.

Effect of boron addition on injection molded 316L stainless steel: mechanical, corrosion properties and in vitro bioactivity.

The research was investigated the effect of boron additions on sintering characteristics, mechanical, corrosion properties and biocompatibility of injection molded austenitic grade 316L stainless steel. Addition of boron is promoted to get high density of sintered 316L stainless steels. The amount of boron plays a role in determining the sintered microstructure and all properties. In this study, 316L stainless steel powders have been used with the elemental NiB powders. A feedstock containing 62.5 wt% powders loading was molded at different injection molded temperature. The binders were completely removed from molded components by solvent and thermal debinding at different temperature. The debinded samples were sintered at different temperature for 60 min. Mechanical property, microstructural characterization and electrochemical property of the sintered samples were performed using tensile testing, hardness, optical, scanning electron microscopy and electrochemical corrosion experiments. Sintered samples were immersed in a simulated body fluid (SBF) with elemental concentrations that were comparable to those of human blood plasma for a total period of 15 days. Both materials were implanted in fibroblast culture for biocompatibility evaluations were carried out. Results of study showed that sintered 316L and 316L with NiB addition samples exhibited high mechanical and corrosion properties in a physiological environment. Especially, 316L with NiB addition can be used in some bioapplications.

Interaction between boron and aluminum and their effects on phenolic metabolism of Linum usitatissimum L. roots.

Aluminum toxicity is the most important limiting factor for plant growth and development in acidic soils (pH < 5.5). Inhibition of root growth has been considered as a sensitive marker of aluminum toxicity and the best indicator of boron deficiency as well. On the other hand cell wall phenolics (lignin and phenolic acids) have important roles in the reduction of extensibility of cell wall under stress conditions. Therefore, the interaction between boron and aluminum on phenolic compounds and the activity of the enzymes involved in their biosynthesis were investigated in flax seedlings. The seedlings were grown in Hoagland's solution and were treated with 3 B levels (4.5, 45 and 450 µM of H3BO3 at deficient, normal, and excess conditions, respectively) and 3 Al levels (0, 50 and 100 µM of AlCl3.6H2O). The results showed that Al treatment did not affect B content of roots (B content of Al-treated and non-treated plants were identical), while B treatment, particularly in higher concentrations, decreased Al content of roots compared with the control plants. In addition, the highest B concentration prevented the inhibitory effect of Al on the root length of plants. High concentrations of B also resulted in the decrease of enzyme activities involved in phenolic compounds (i.e., phenylalanine ammonia-lyase, polyphenol oxidase and peroxidase), decrease of lignin content and wall-bound phenols under Al stress, thereby ameliorating Al toxicity. The results suggest that the requirement of flax plants for B under Al stress conditions is higher than that required for growth in normal conditions without Al.

Carborane derivatives loaded into liposomes as efficient delivery systems for boron neutron capture therapy.

Boron neutron capture therapy (BNCT) is an anticancer therapy based on the incorporation of (10)B in tumors, followed by neutron irradiation. Recently, the synthesis and delivery of new boronated compounds have been recognized as some of the main challenges in BNCT application. Here, we report on the use of liposomes as carriers for BNCT active compounds. Two carborane derivatives, i.e., o-closocarboranyl beta-lactoside (LCOB) and 1-methyl-o-closocarboranyl-2-hexylthioporphyrazine (H(2)PzCOB), were loaded into liposomes bearing different surface charges. The efficacy of these formulations was tested on model cell cultures, that is, DHD/K12/TRb rat colon carcinoma and B16-F10 murine melanoma. These induce liver and lung metastases, respectively, and are used to study the uptake of standard BNCT drugs, including borophenylalanine (BPA). Boron concentration in treated cells was measured by alpha spectrometry at the TRIGA mark II reactor (University of Pavia). Results showed high performance of the proposed formulations. In particular, the use of cationic liposomes increased the cellular concentration of (10)B by at least 30 times more than that achieved by BPA.

Histomorphometric study of alveolar bone healing in rats fed a boron-deficient diet.

Bone healing after tooth extraction in rats is a suitable experimental model to study bone formation. Thus, we performed a study to determine the effects of boron (B) deficiency on bone healing by using this model. The first lower right molar of weanling Wistar rats was extracted under anesthesia. The animals were divided into two groups: +B (adequate; 3 mg B/kg diet), and -B (boron-deficient; 0.07 mg/kg diet). The animals in both groups were killed in groups of 10 at 7 and 14 days after surgery. The guidelines of the NIH for the care and use of laboratory animals were observed. The mandibles were resected, fixed, decalcified, and embedded in paraffin. Buccolingually oriented sections were obtained at the level of the mesial alveolus and used for histometric evaluations. Total alveolar volume (TAV) and trabecular bone volume per total volume (BV/TV) in the apical third of the alveolus were determined. Percentages of osteoblast surface (ObS), eroded surface (ES), and quiescent surface (QS) were determined. No statistical significant differences in food intake and body weight were observed. Histomorphometric evaluation found -B rats had 36% and 63% reductions in BV/TV at 7 and 14 days, respectively. When compared with +B rats, -B rats had significant reductions (57% and 87%) in ObS concomitantly with increases (120% and 126%) in QS at 7 and 14 days, respectively. The findings show that boron deficiency results in altered bone healing because of a marked reduction in osteogenesis.

Novel inhibitor for prolyl tripeptidyl aminopeptidase from Porphyromonas gingivalis and details of substrate-recognition mechanism.

A new inhibitor, H-Ala-Ile-pyrrolidin-2-yl boronic acid, was developed as an inhibitor against prolyl tripeptidyl aminopeptidase with a K(i) value of 88.1 nM. The structure of the prolyl tripeptidyl aminopeptidase complexed with the inhibitor (enzyme-inhibitor complex) was determined at 2.2 A resolution. The inhibitor was bound to the active site through a covalent bond between Ser603 and the boron atom of the inhibitor. This structure should closely mimic the structure of the reaction intermediate between the enzyme and substrate. We previously proposed that two glutamate residues, Glu205 and Glu636, are involved in the recognition of substrates. In order to clarify the function of these glutamate residues in substrate recognition, three mutant enzymes, E205A, E205Q, and E636A were generated by site-directed mutagenesis. The E205A mutant was expressed as an inclusion body. The E205Q mutant was expressed in soluble form, but no activity was detected. Here, the structures of the E636A mutant and its complex with the inhibitor were determined. The inhibitor was located at almost the same position as in the wild-type enzyme-inhibitor complex. The amino group of the inhibitor interacted with Glu205 and the main-chain carbonyl group of Gln203. In addition, a water molecule in the place of Glu636 of the wild-type enzyme interacted with the amino group of the inhibitor. This water molecule was located near the position of Glu636 in the wild-type and formed a hydrogen bond with Gln203. The k(cat)/K(M) values of the E636A mutant toward the two substrates used were smaller than those of the wild-type by two orders of magnitude. The K(i) value of our inhibitor for the E636A mutant was 48.8 microM, which was 554-fold higher than that against the wild-type enzyme. Consequently, it was concluded that Glu205 and Glu636 are significant residues for the N-terminal recognition of a substrate.

Absorption of boron after mouthwash treatment with Bocosept.

Healthy volunteers and patients with gingivitis were treated locally with the boron-containing bacteriostatic agent, Bocosept. Blood levels and urinary excretion of boron were examined by a spectrophotometric method. Blood concentrations after a single mouthwash with Bocosept slightly exceeded those after intake of 200 g raisins or a bottle of red wine. The blood levels during a one-week course of treatment showed a low rate of boron accumulation. The highest concentration was about 0.3 microgram B/ml, a level which does not seem to involve any risk of boron poisoning. The small amount taken up after mouthwash treatment with Bocosept does not appear to represent absorption by the oral mucosa. It seems more likely that the uptake of boron takes place in the intestine after ingestion of residual amounts from the mouth.

Formation of rhamnogalacturonan II-borate dimer in pectin determines cell wall thickness of pumpkin tissue.

Boron (B) deficiency results in inhibition of pumpkin (Cucurbia moschata Duchesne) growth that is accompanied by swelling of the cell walls. Monomeric rhamnogalacturonan II (mRG-II) accounted for 80% to 90% of the total RG-II in B-deficient walls, whereas the borate ester cross-linked RG-II dimer (dRG-II-B) accounted for more than 80% of the RG-II in control plants. The results of glycosyl residue and glycosyl linkage composition analyses of the RG-II from control and B-deficient plants were similar. Thus, B deficiency does not alter the primary structure of RG-II. The addition of (10)B-enriched boric acid to B-deficient plants resulted within 5 h in the conversion of mRG-II to dRG-II-(10)B. The wall thickness of the (10)B-treated plants and control plants was similar. The formation and possible functions of a borate ester cross-linked RG-II in the cell walls are discussed.