Interaction of lysozyme with a surface protein antigen of Streptococcus mutans

Assessment of biofilm formation on ceramic, metal, and plastic brackets in orthodontic materials by new method using renG-expressing Streptococcus mutans

OBJECTIVE

Oral biofilm has a high acid-producing capacity, increases the risk of enamel demineralization around brackets, and has been identified as a problem in orthodontic treatment. Here, we assessed the risk of biofilm formation by Streptococcus mutans, which is associated with the development of white spot lesions (WSL) on tooth surfaces, using multibracket devices.

METHODS

Various types of brackets were used for the biofilm formation assay with S. mutans coated with human saliva, immersed in renG-expressing S. mutans UA159 (strain with the luciferase gene inserted), and incubated overnight at 37 °C under aerobic conditions containing 5% CO2. The biofilm was washed twice with phosphate-buffered saline (PBS), and 200 μL of luciferin dissolved in PBS was added to each well. The mixture was light shielded and allowed to react for 20 min. Luminescence was measured as the amount of biofilm formed by live cells on the bracket surfaces using an optical emission spectrophotometer.

RESULTS

Biofilm formation was greater in plastic brackets than in ceramic and metal brackets in a number-dependent manner. However, biofilm formation was inhibited as the plastic bracket was coated with saliva.

CONCLUSION

For preventive treatments of WSL onset during orthodontic treatment, orthodontists should carefully select and customize brackets based on patient needs, goals, and biomechanical principles. This study developed a new measurement method using renG-expressing S. mutans UA159 to accurately assess active biofilm formation on bracket surfaces.

Elucidation of the Binding Interaction between β-sitosterol and Lysozyme using Molecular Docking, Molecular Dynamics and Surface Plasmon Resonance Analysis

In this study, the binding behavior of β-sitosterol with lysozyme (LZM) was elucidated by surface plasmon resonance (SPR), computational molecular docking and molecular dynamics simulation studies. Chicken egg white lysozyme (CEWLZM) served as a model protein. Tri-N-acetylchitotriose (NAG3) was used in the redocking experiments to generate precise binding location of the protein. β-sitosterol displayed a slightly better binding energy (-6.68±0.04 kcal/mol) compared to NAG3. Further molecular dynamics simulations and MMPBSA analysis revealed that residues Glu35, Gln57-Asn59, Trp62, Ile98, Ala107 and Trp108 contribute to the binding energy. Then, 2.5 mg/mL CEWLZM, 1X PBS buffer (pH 7.4) as running and coupling buffers, 30 μL/min as flow rate were applied for SPR analysis. Serial β-sitosterol injections (20-150 μM) were performed through SPR sensor surface. According to SPR binding study, KD value for β-sitosterol-CEWLZM binding interaction was calculated as 71.34±9.79 μM. The results could provide essential knowledge for nutrition, pharmaceutical science, and oral biology.

Antibacterial and Antibiofilm Photodynamic Activities of Lysozyme-Au Nanoclusters/Rose Bengal Conjugates

Antibacterial photodynamic therapy (aPDT) utilizes reactive oxygen species such as singlet oxygen (1O2) and free radicals via photosensitizers, which are light and light-sensitive agents, to reduce bacterial infections. It has been utilized as a treatment for dental diseases in place of antibiotic therapies. However, aPDT does not always cause the desired therapeutic effect due to the instability of organic photosensitizers and the formation of bacterial biofilms. To promote the antibacterial and antibiofilm effects of aPDT, we have proposed a lysozyme (Lys)-gold nanoclusters (Au NCs)/rose bengal (Lys-Au NCs/RB) conjugate as a novel photosensitizer. This conjugate was found to effectively impede the growth of both gram-positive and gram-negative bacteria when exposed to white light-emitting diode (LED) irradiation. The photoexcited Lys-Au NCs/RB showed significantly higher antibacterial activity than photoexcited Lys-Au NCs or RB alone. The synergistic effect is a result of the combination of Lys (an antibacterial protein) and enhanced 1O2 generation related to resonance energy transfer (RET) in the Au NCs/RB conjugate. Photoexcited Lys-Au NCs/RB increased the effects of aPDT in a dose- and time-dependent manner. Furthermore, the photoexcited Lys-Au NCs/RB successfully decreased Streptococcus mutans biofilm formation. However, in contrast, it did not have a negative effect on the proliferation, adhesion, or spread of mammalian cells, indicating low cytotoxicity. Lys-Au NCs/RB is a novel photosensitizer with low cytotoxicity that is capable of bacterial inactivation and the suppression of biofilm formation, and could help to improve dental treatments in the future.

Roles of membrane vesicles from Streptococcus mutans for the induction of antibodies to glucosyltransferase in mucosal immunity

Glucosyltransferase (Gtf) B and GtfC from Streptococcus mutans are key enzymes for the development of biofilm-associated diseases such as dental caries. Gtfs are involved in membrane vesicles (MVs) and function in the formation of biofilms by initial colonizers such as Streptococcus mitis and Streptococcus oralis on the tooth surface. Therefore, MVs may be important virulence factors and targets for the prevention of biofilm-associated disease. To clarify how GtfB encoded by gtfB and GtfC encoded by gtfC associate with MVs and whether MVs are effective as a mucosal immunogen to induce the production of antibodies against Gtfs, MVs from S. mutans UA159 wild-type (WT), gtfB^-, gtfC^- and gtfB^-C^- were extracted from culture supernatants by ultracentrifugation and observed by scanning electron microscopy. Compared with GtfB, GtfC was mainly contained in MVs and regulated the size and aggregation of MVs, and the biofilm formation of S. mutans. The intranasal immunization of BALB/c mice with MVs plus a TLR3 agonist, poly(I-C), was performed 2 or 3 times for 5 weeks, with an interval of 2 or 3 weeks. MVs from all strains caused anti-MV IgA and IgG antibody production. In quality analysis of these antibodies, the IgA and IgG antibodies produced by immunization with MVs from WT and gtfB^- strains reacted with Gtfs in the saliva, nasal wash and serum but those produced by immunization with MVs from gtfC^- and gtfB^-C^- strains did not. S. mutans MVs mainly formed by GtfC are an intriguing immunogen for the production of anti-Gtf antibodies in mucosal immunogenicity.

Antimicrobial Capacity of Casein Phosphopeptide/Amorphous Calcium Phosphate and Enzymes in Glass Ionomer Cement in Dentin Carious Lesions

OBJECTIVE

To evaluate the ability of casein phosphopeptide/amorphous calcium phosphate (CPP/ACP) and lysozyme, lactoferrin, and lactoperoxidase (LLL) added to glass ionomer cement (GIC) to inhibit the growth of S. mutans in a caries model.

MATERIAL AND METHODS

Eighty permanent third molars were selected. The dentin of these teeth was exposed and flattened. Except for the coronal dentin, the specimens were waterproofed, autoclaved, and submitted to cariogenic challenge with standard strain of S. mutans. The carious lesions were sealed as follows: group 1 (n=20): GIC without additives; group 2 (n=20): GIC + CPP/ACP; group 3 (n=20): GIC + LLL; group 4 (n=20): GIC + CPP/ACP + LLL. S. mutans counts were performed before the caries were sealed (n=5), after 24 hours (n=5), at 1 month (n=5), and at 6 months (n=5). The results were analyzed using descriptive statistical analysis and the Kruskal-Wallis test (Student-Newman-Keuls test).

RESULTS

GIC + LLL caused a significant reduction of S. mutans 1 month after sealing (p<0.01); however, there was a significant growth of S. mutans 6 months after sealing. GIC, GIC + CPP/ACP, and GIC + CPP/ACP + LLL showed similar behavior with significant reduction of S. mutans after 24 hours (p<0.05) and increase after 1 and 6 months.

CONCLUSION

The addition of LLL to GIC increases the antimicrobial action of GIC on S. mutans. This leads to control of bacterial biofilm for 1 month, thus stopping the progression of carious lesions.

Rapid detection of S. mutans surface antigen I/II using a sensitive monoclonal anti-Ag I/II antibody by ELISA

The cell-surface protein antigen I/II (Ag I/II) is expressed in oral streptococci, which are known as the causative agent of a number of diseases including dental caries, endocarditis, gingivitis, and periodontal disease. Consequently, monoclonal antibodies (MAb) capable of recognizing the streptococcal Ag I/II protein could be a useful tool for the diagnosis and cure of these diseases. In this study, a previously generated monoclonal anti-Ag I/II antibody, ckAg I/II, was used to detect a small amount of Streptococcus mutans (S. mutans) surface antigen Ag I/II. The ckAg I/II was proved to be very sensitive and able to detect as little as 1 ng of recombinant Ag I/II protein within 5 min and Ag I/II in saliva within 10 min, as well as native Ag I/II in 20 μL of culture supernatant by ELISA. These results suggest that ckAg I/II can be used as a fast and efficient diagnostic tool to detect Ag I/II.

Effect of nanogrinding on the pigment and bioactivity of Djulis ( Chenopodium formosanum Koidz.)

Betanin is an antioxidant pigment found in djulis, a grain native to Taiwan, and is a natural source food coloring, but the bright red color degrades rapidly if submitted to light, heat, or oxygen. The effects of nanogrinding on the stability of pigments and bioactive components are unknown. In this study, the color characteristics and bioactivity (antioxidant capacity and enzyme activity) of nanoparticle (NP) djulis was compared with those of intact granules (IG) and microparticles (MP). Results showed that the NP samples exhibited the highest betanin content (2.04 mg/g), which was almost twice that of IG. It was observed that nanogrinding resulted in higher pigment extraction efficiency. However, during storage (5-35 °C for 56 days), NP samples showed the most serious pigment degradation, and this color degradation, as expected, had the lowest activation energy. This was more evident when the storage temperature was high. Antioxidant capacities showed the same trends. MP and NP exhibited significantly higher activity of superoxide dismutaste-like activity, lactoperoxidase (LPO), and lysozyme than IG. Gel permeation chromatography confirmed the degradation of larger particles during nanogrinding, which might favor enzyme extraction and their activities. Statistical analysis revealed a close relationship between betanin and antioxidant capacity.

The role of anti-PAc (361-386) peptide SIgA antibody in professional oral hygiene of the elderly

OBJECTIVE

Measurement of salivary IgA antibody (PAc-peptide antibody, PPA) to amino acid residues 361-386 of Streptococcus mutans PAc, which possess a multiple binding motif to various HLA-DR molecules and a B-cell epitope that recognises the inhibiting antibody to S. mutans, is an indicator for the population numbers of mutans streptococci (MS) in human saliva. The purpose of this study was to clarify the role of PPA in infection control of MS after professional oral hygiene care.

MATERIALS AND METHODS

Thirty-nine dependently living institutionalised elderly subjects (75.9 +/- 7.5 years; 10 males, 29 females) participated in the study. The measurements of PPA, MS, total streptococci (TS) and lactobacilli (LB) were performed by ELISA and culture techniques from saliva, plaque and tongue samples from the elderly.

RESULTS

After treatment using professional oral care, the numbers of MS decreased significantly at 6 months in saliva and tongue samples from the group not having PPA in comparison with the primary data; whereas in the PPA-detected group, a significant decrease in MS number was shown immediately following professional care at 1-12 months in all samples. There was little difference in the numbers of LB at any of the time points. The numbers of TS decreased rapidly in PPA-not detected group in comparison with the PPA-detected group.

CONCLUSION

PPA may be more effective for controlling MS number in the oral cavity after professional treatment. The measurement of PPA may be used for preventive instruction to dental caries at the chair side in the clinical setting.

Relationships of anti-PAc (361-386) peptide salivary IgA antibody, eosinophils and basophils with periodontal status in the elderly

The amino acid residues 361-386 of Streptococcus mutans PAc includes an important region associated with the interaction between S. mutans and salivary components. We investigated the relationships between levels of the anti-PAc (361-386) peptide antibody (PPA) in saliva and periodontal status in 281 elderly subjects (mean age 77 years; 118 females, 163 males) by assessing dental calculus (CA), attachment loss (AL), pocket depth (PD), bleeding on probing (BOP) and various blood parameters. Enzyme-linked immunosorbent assay results revealed that subjects with a PPA level of greater than 0.1 (PPA detected group) showed a lower average value for number of sites with more than 6 mm of AL/6 points x 100/tooth (rAL6) than those with a PPA level of less than 0.1 (PPA not detected group). Furthermore, average values for rAL6 were significantly lower in the PPA detected group, and BOP, AL and rAL6 correlated positively and significantly with the percentage of eosinophils present in leukocytes in female subjects in both groups. PPA level had a negative correlation with percentages of basophils and eosinophils. The results indicate that systemic increases in numbers of eosinophils and basophils are associated with the development of periodontal diseases, while PPA level may be a useful indicator of periodontal status.

Role of peptide antigen for induction of inhibitory antibodies to Streptococcus mutans in human oral cavity

The alanine-rich repeating region (A-region) in the surface protein antigen (PAc) of Streptococcus mutans has received much attention as an antigenic component for vaccines against dental caries. The PAc (residue 361-386) peptide in the A-region possesses a multiple binding motif (L- -V-K- -A) to various HLA-DR molecules and a B-cell core epitope (- Y- - -L- -Y- - - -) that recognizes the inhibiting antibody to S. mutans. In the present study, we investigated the immunogenicity of the PAc (361-386) peptide in humans and regulators of induction of the anti-PAc (361-386) peptide IgA antibody (aPPA) in saliva. The PAc (361-386) peptide was confirmed as an ideal peptide antigen for induction of the inhibiting antibody to S. mutans in 151 healthy human subjects (36.6 +/- 12.6 years old) by quantitative analyses of oral bacteria and ELISA, as the aPPA titre in human saliva decreased significantly in an age-dependent manner. Homozygous DRB1*0405 and 1502, and heterozygous DRB1*0405/1502 showed a negative association with production of aPPA and tended to reduce the number of total streptococci in saliva. In contrast, the DRB1*1501 allele was significantly correlated with a high level of induction of the antibodies, and also tended to reduce lactobacilli and mutans streptococci. Further, peptide immunogenicity was confirmed in NOD-SCID mice grafted with human peripheral blood mononuclear cells. Our results indicate that the interplay between regulators such as age, DRB1 genotype, cytokines, and peptide immunogenicity may provide a potential means for developing a vaccine useful for the prevention of dental caries as well as their diagnosis.

Molecular interactions of surface protein peptides of Streptococcus gordonii with human salivary components

Oral streptococci play a large role in dental biofilm formation, and several types interact as early colonizers with the enamel salivary pellicle to form the primary biofilm, as well as to incorporate other bacteria on tooth surfaces. Interactions of surface molecules of individual streptococci with the salivary pellicle on the tooth surface have an influence on the etiological properties of an oral biofilm. To elucidate the molecular interactions of streptococci with salivary components, binding between surface protein (SspB and PAg) peptides of Streptococcus gordonii and Streptococcus sobrinus were investigated by utilizing BIAcore biosensor technology. The analogous peptide [change of T at position 400 to K in SspB(390-402), resulting in the SspB(390-T400K-402) peptide] from S. gordonii showed the greatest response for binding to salivary components and inhibited the binding of Streptococcus sanguis by more than 50% in a competitive inhibition assay in a comparison with other SspB and PAg peptides. This peptide also bound to the high-molecular-weight protein complex of salivary components and the agglutinin (gp340/DMBT1) peptide (scavenger receptor cysteine-rich domain peptide 2 [SRCRP 2]). In addition, the SspB(390-T400K-402) peptide was visualized by two surface positive charges in connection with the positively charged residues, in which lysine was a key residue for binding. Therefore, the region containing lysine may have binding activity in S. gordonii and S. sanguis, and the SRCRP 2 region may function as a receptor for the binding. These findings may provide useful information regarding the molecular mechanism of early biofilm formation by streptococci on tooth surfaces.

Real-time interaction of oral streptococci with human salivary components

Oral streptococci are present in large numbers in dental plaque and several types interact with the enamel salivary pellicle to form a biofilm on tooth surfaces. The respective affinity of individual streptococci for salivary components has an influence on the etiologic properties of oral biofilm in the development of dental caries. We studied real-time biospecific interactions between oral streptococci and salivary components utilizing biosensor technology to analyze surface plasmon resonance. Streptococcus sanguis and Streptococcus mutans showed significant responses for binding to salivary components, in comparison with other bacteria. Further, the association rates (4.1 x 10-11/bacterium) and dissociation rate (5.7 +/- 0.9 x 10-3 Second(s)-1) were higher for S. sanguis than for S. mutans (2.4 x 10-11 and 2.9 +/- 0.8 x 10-3) and Streptococcus mitis (1.3 x 10-11 and 3.5 +/- 1.3 x 10-3). However, the association equilibrium constants (8.2 S/bacterium) for S. mutans was 2 times higher in than S. mitis (3.8) and slightly higher than S. sanguis (7.2). These findings may provide useful information regarding the mechanism of early biofilm formation by streptococci on the tooth surface.

Experimental study and mathematical modeling of the interaction between antibodies and antigens on the surface of liposomes

Unilamellar liposomes with incorporated hapten-phospholipid conjugates were proposed as models of polyvalent antigens with migrating determinants for quantitative analysis of their interaction with antibodies. The monovalent pesticide atrazine was used as a model antigen. For its incorporation into the lipid bilayer, the atrazine carboxylated derivative was conjugated with dimyristoylphosphatidylethanolamine (DMPE). Unilamellar liposomes were prepared with dimyristoylphosphatidylcholine/atrazine-DMPE at molar ratios of 90:10, 95:5, 98:2, 99:1 and 99.5:0.5. Their interaction with the peroxidase-labeled anti-atrazine antibodies was studied by enzyme immunoassay and polarization fluoroimmunoassay techniques. It was shown that the increase in hapten content in the liposomes from 0.5 to 10 mol% led to an increase in the equilibrium constants of the interaction with antibodies from 0.093 x 10(8) to 0.303 x 10(8)M-1. The association rate constants varied from 1.45 x 10(5) to 15.5 x 10(5)M-1 s-1 depending on the antigen content in liposomes and experimental conditions. The measured constants were applied for a mathematical model describing multi-step interaction between antibodies and polyvalent liposomal antigens. The model adequately describes the quantitative regularities of the influence of antigen content and the affinity of immunochemical interaction on the quantity and the dynamics of the immune complexes forming.

Identification of the peptide motifs that interact with HLA-DR8 (DRB1*0802) in Streptococcus mutans proteins

A glucosyltransferase (GTF) and a surface protein antigen (PAc) of Streptococcus mutans have been suggested as possible components of an effective dental caries vaccine. To identify antigenic peptides in GTF and PAc that bind to MHC class II (HLA-DR8, DRB1*0802) molecules, we investigated binding activities to DR8 molecules of overlapping synthetic peptides at several sites in GTF and in the alanine-rich repeating region of PAc using an ELISA-inhibition competitive binding assay for the interaction between the HLA-DR molecule and the PAc (316-334) peptide. Six GTF peptides and 10 PAc peptides strongly bound to the HLA-DR8 molecule. In a homology analysis of the amino acid sequences of the six GTF peptides, two binding motifs were found in L/Y--Y/L-A/N and Y/L--N/G/E--Y-V/L/P. Moreover, a new binding motif in PAc was found in L--Y-A. It is suggested that these binding motifs could be useful in designing a dental caries vaccine in humans.

Inhibitory effect of bovine milk lactoferrin on the interaction between a streptococcal surface protein antigen and human salivary agglutinin

Human whole saliva induces aggregation of Streptococcus mutans cells via an interaction between a surface protein antigen (PAc) of the organism and salivary agglutinin. Bovine milk inhibits the saliva-induced aggregation of S. mutans. In this study, the milk component that possesses inhibitory activity against this aggregation was isolated and found to be lactoferrin. Surface plasmon resonance analysis indicated that bovine lactoferrin binds more strongly to salivary agglutinin, especially to high molecular mass glycoprotein, which is a component of the agglutinin, than to recombinant PAc. The binding of bovine lactoferrin to salivary agglutinin was thermostable, and the optimal pH for binding was 4.0. To identify the saliva-binding region of bovine lactoferrin, 11 truncated bovine lactoferrin fragments were constructed. A fragment corresponding to the C-terminal half of the lactoferrin molecule had a strong inhibitory effect on the saliva-induced aggregation of S. mutans, whereas a fragment corresponding to the N-terminal half had a weak inhibitory effect. Seven shorter fragments corresponding to lactoferrin residues 473-538 also showed a high ability to inhibit the aggregation of S. mutans. These results suggest that residues 473-538 of bovine lactoferrin are important in the inhibition of saliva-induced aggregation of S. mutans.

Identification of Streptococcus mutans PAc peptide motif binding with human MHC class II molecules (DRB1*0802, *1101, *1401 and *1405)

A surface protein antigen (PAc) of Streptococcus mutans, in particular the A-region of this PAc molecule, has been noted as a possible target in research for an effective dental caries vaccine. To identify the antigenic peptide binding to major histocompatibility complex (MHC) class II (HLA-DR) molecules in the A-region, we prepared a panel of overlapping synthetic peptides in the second unit of the A-region, and established that a simple enzyme-linked immunosorbent assay (ELISA) binding assay could be achieved by incubating the DR-crude. Binding to DR molecules of these peptides from nine donors was investigated by using the ELISA binding assay. It was revealed that the PAc(316-334) peptide bound more strongly to the HLA-DR molecule in seven out of nine subjects. In particular, DR8 (DRB1*0802), DR5 (DRB1*1101) and DR6 (DRB1*1402 and *1405), which bound strongly to PAc(316-334) peptide, were identified. Moreover, we synthesized glycine-substituted peptide analogues of the peptide and examined the binding motif of the binding region. As a result, the multiple binding motif in DR8, DR5 and DR6 was found in L-RV-K-A. It is suggested that a peptide vaccine for dental caries that is more effective for humans, with fewer adverse side-effects, could be designed by combining the multiple binding motif with the B-cell epitope to produce only the inhibiting antibody against dental caries. The peptide could therefore be useful for peptide vaccine development in the general human population.

Binding of salivary glycoprotein-secretory immunoglobulin A complex to the surface protein antigen of Streptococcus mutans

The interaction between a surface protein antigen (PAc) of Streptococcus mutans and human salivary agglutinin was analyzed with a surface plasmon resonance biosensor. The major component sugars of the salivary agglutinin were galactose, fucose, mannose, N-acetylglucosamine, N-acetylgalactosamine, and N-acetylneuraminic acid. Binding of salivary agglutinin to PAc was calcium dependent and heat labile and required a pH greater than 5. Binding was significantly inhibited by N-acetylneuraminic acid and alpha2,6-linked sialic acid-specific lectin derived from Sambucus sieboldiana in a dose-dependent manner. Pretreatment of the salivary agglutinin with sialidase reduced the binding activity of the agglutinin to the PAc molecule. The agglutinin was dissociated into high-molecular-mass glycoprotein and secretory immunoglobulin A (sIgA) components by electrophoretic fractionation in the presence of 1% sodium dodecyl sulfate and 1% 2-mercaptoethanol. Neither of the components separated by electrophoretic fractionation, high-molecular-mass glycoprotein or sIgA, bound to the PAc molecule. Furthermore, the high-molecular-mass glycoprotein strongly inhibited the binding of the native salivary complex to PAc. These results suggest that the complex formed by the high-molecular-mass salivary glycoprotein and sIgA is essential for the binding reaction and that the sialic acid residues of the complex play an important role in the interaction between the agglutinin and PAc of S. mutans.

Identification of core B cell epitope in the synthetic peptide inducing cross-inhibiting antibodies to a surface protein antigen of Streptococcus mutans

A surface protein antigen (PAc) of Streptococcus mutans, in particular, A-region of the molecule, has been considered as a possible target for the development of an effective anticaries vaccine. This region might be implicated in the induction of dental caries via interaction with salivary components. We have recently specified a unique peptide, TYEAALKQYEADL, as one of the minimum peptides that completely corresponds to the amino acid sequence of a part of the A-region. The unique peptide contains both T and B cell epitopes for the induction of cross-reacting antibodies to the PAc. In this study, we synthesized valine or glycine-substituted peptide analogs of this peptide and examined core B cell epitopes of this unique peptide by using ELISA inhibition assay. As a result, the core amino acid residues of -Y------Y---- for B cell recognition were found to likely be not only important amino acids stabilizing the structure, but also might be essential for induction of the cross-inhibiting antibodies against PAc. These results will hopefully provide us with useful information for the design of an effective anticaries peptide vaccine.

Alkaline phosphatase-Strep tag fusion protein binding to streptavidin: resonant mirror studies

The properties of a fusion protein comprising a streptavidin recognition sequence (Strep tag) fused to the C terminus of Escherichia coli alkaline phosphatase are described. The catalytic properties were determined with p-nitrophenyl phosphate and compared to those of the native E. coli alkaline phosphatase. It was found that the Km values were similar in both cases (8 microM for transferase and 2 microM for hydrolase activities) whilst the Vmax values were lower for the fusion protein, possibly due to the presence of misfolded forms. An optical biosensor based on the resonant mirror was used to determine the binding kinetics between the fusion protein and the immobilised streptavidin. The association and dissociation rate constants were determined to be 2.1(+/-0.3) x 10(-2) microM(-1) s(-1) and 11(+/-0.2) x 10(-3) s(-1), respectively, which results in an equilibrium dissociation constant of 0.5 microM. This is larger than previously reported affinities based on titration calorimetry and may be a consequence of the presence of two streptavidin binding sequences on the dimeric alkaline phosphatase simultaneously binding to two subunits of streptavidin.