Capillary electrochromatography of peptides in a microfabricated system

Reversed-phase liquid chromatography of tryptic peptides is shown in the capillary electrochromatography mode using microfabricated columns. Although selectivity is different, a mixture of tryptic peptides from ovalbumin appears to be as easily separated in the CEC as HPLC mode. The major difference between a separation in the macrofabricated CEC column and conventional separations in the HPLC mode is that separations are more readily achieved in the isocratic mode in the lower surface area microfabricated CEC columns.

P300: the similarities and differences in the scalp distribution of visual and auditory modality

PURPOSE

To examine the topographic relationship of P3(00) between the visual and auditory modalities, especially to examine whether there are any modality-specific hemispheric differences of P3 in normal adults.

METHODS

The P3s were recorded from the same 41 normal right-handed males between the ages of 20 and 33 in both a typical auditory oddball task and a visual oddball paradigm with novel stimuli, with an extensive set of 61 scalp electrodes. In addition to the visual comparison and quantitative assessment of current source density (CSD) maps between the two modalities, canonical correlation analyses on the P3 raw amplitudes and examination of interaction effects of modality x location on both raw and normalized P3 data were performed.

RESULTS

The canonical correlation between modalities was generally high, especially at the left parietal brain region. There were no significant hemispheric effects in anterior brain but significant left-greater-than-right hemispheric effects in posterior brain regions in both modalities; modality-specific hemispheric effect was observed only at the parietal region. Strong surface current density activities were observed in the midline parietal-occipital area, and left and right boundary areas of temporal and inferior frontal region.

CONCLUSIONS

The topographic similarities between P3s recorded in the visual and auditory modality outnumber the differences. Combining data from CSD assessments and profile analysis of P3 topography support the hypothesis of multiple generators of P3 that are differentially active in processing stimuli from different sensory modalities and are not symmetrically distributed between the two hemispheres.

Tumor necrosis factor-alpha stimulates lactate dehydrogenase A expression in porcine cultured sertoli cells: mechanisms of action

In the present study, we investigated the regulatory action of tumor necrosis factor-alpha (TNFalpha) on lactate dehydrogenase A (LDH A), a key enzyme involved in lactate production. To this end, use was made of a primary culture system of porcine testicular Sertoli cells. TNFalpha stimulated LDH A messenger RNA (mRNA) expression in a dose (ED50 = 2.5 ng/ml; 0.1 nM TNFalpha)-dependent manner. This stimulatory effect was time dependent, with an effect detected after 6 h of TNFalpha treatment and maximal after 48 h of exposition (5-fold; P<0.001). The direct effect of TNFalpha on LDH A mRNA could not be accounted for by an increase in mRNA stability (half-life = 9 h), but was probably due to an increase in LDH A gene transcription. Inhibitors of protein synthesis (cycloheximide), gene transcription (actinomycin D and dichlorobenzimidazole riboside), tyrosine kinase (genistein), and protein kinase C (bisindolylmaleimide) abrogated completely (actinomycin D, dichlorobenzimidazole riboside, cycloheximide, and genistein) or partially (bisindolylmaleimide) TNFalpha-induced LDH A mRNA expression. These observations suggest that the stimulatory effect of TNFalpha on LDH A mRNA expression requires protein synthesis and may involve a protein tyrosine kinase and protein kinase C. In addition, we report that LDH A mRNA levels were increased in Sertoli cells treated with FSH. However, although the cytokine enhances LDH A mRNA levels through increased gene transcription, the hormone exerts its stimulatory action through an increase in LDH A mRNA stability. The regulatory actions of the cytokine and the hormone on LDH A mRNA levels and therefore on lactate production may operate in the context of the metabolic cooperation between Sertoli and postmeiotic germ cells in the seminiferous tubules.

[A study of p53 gene mutations in squamous cell carcinoma in eyes]

OBJECTIVE

To inquire into the molecular biological mechanisms of the occurrence of squamous cell carcinoma in eyes.

METHODS

Using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method, we detected 63 samples taken from 63 cases (including 36 invasive squamous carcinoma, 7 carcinoma in situ, 20 papillomas) for 248 point mutations in p53 gene.

RESULTS

22.2% (8/36) squamous cell carcinomas, 14.3% (1/7) carcinomas in situ were shown to contain the point mutation in codon 248 of p53 gene exon 7 and no point mutation was found in papillomas, the difference being significant (P = 0.021). No point mutation was in patients over 70, while in patients younger than 70, the mutation rate was 33.3%, the difference being significant (P = 0.24). No significant correlation was found between p53 gene mutations and clinical pathological manifestations (P > 0.05).

CONCLUSIONS

p53 gene mutations play roles in the early stage of the development of squamous cell carcinomas, and there is no correlation between p53 gene mutations and clinical manifestations. The mutation in p53 gene is not the main cause of neoplasms in the elder persons after the seventh decade of life.

Event-related potential index of semantic mnemonic dysfunction in abstinent alcoholics

BACKGROUND

The objective of the study was to expand the investigation of the match/mismatch mnemonic impairment in the semantic domain in sober alcoholics.

METHODS

Event-related potentials (ERPs) were recorded from 28 healthy adults and 36 sober alcoholics in a category (either animals or fruits/vegetables) match/nonmatch S1-S2 paradigm.

RESULTS

There was a significant interaction of ERP amplitude (c3) between groups (controls vs. alcoholics) and stimulus conditions (category match vs. nonmatch) at the posterior brain regions; the c3 component was smaller for the category match than for nonmatch trials in controls, with the absence of such c3 differences in alcoholics. There were no significant ERP differences between the two groups in processing the sample stimuli. The ERPs c2) elicited by the animal category were larger than those for the vegetable category in both groups. The alcoholics showed prominent suppressed activation of left temporooccipital brain regions under both matching and nonmatching conditions, as demonstrated by the current source density maps. The alcoholics were also slower and less accurate than the controls in judging both category matching and nonmatching stimuli, while neither of the two groups demonstrated shorter response times to the matching stimuli.

CONCLUSIONS

These data suggest that alcoholics are less efficient in the semantic mnemonic match/nonmatch process, and are less likely to be deteriorated in the stage of forming the template for such match/nonmatch comparisons.

[A study on expressions of P53 protein and proliferating cell nuclear antigen in malignant eyelid tumors]

OBJECTIVE

To observe the relationship between P53 gene mutation and malignant eyelid tumor.

METHODS

The expressions of P53 protein and proliferating cell nuclear antigen (PCNA) in 102 cases with malignant eyelid tumors were examined by labelled streptravidin biotin (LSAB) immunohistochemical technique.

RESULTS

Although overexpression of P53 protein was not detected in normal eyelid cutaneous epithelium, meibomian gland and mild epithelial dysplasia, the positive rate of P53 increased in degree in moderate and severe dysplasia, carcinoma in situ and invasive tumors, and PCNA labeling index (PI) progressively increased too. Overexpression of P53 protein was detected in 30/53 cases of basal cell carcinoma (BCC), 16/32 cases of squamous cell carcinoma (SCC), 10/17 cases of meibomian gland carcinoma, and the intensity of P53 protein immunostaining increased with the lowering in the degree of histological differentiation. The mean PCNA labeling index was 38.1 +/- 31.1, 38.3 +/- 30.1, 42.6 +/- 32.0 respectively in BCC, SCC and meibomian gland carcinoma. The PI value in tumor tissue with positive P53 protein expression was higher than that in tissues with its negative expression (P < 0.05).

CONCLUSIONS

The gene mutation of P53 protein might play an important role in the pathogenesis of malignant eyelid tumors, it may be one of the factors that can lead to abnormal hyperactivity of cell proliferation and it possibly can be a malignant sign in the progression of a tumor.

Event-related potentials during digit recognition tasks

OBJECTIVE

An event-related potential (ERP) correlate of visual short-term memory (VMP) has been identified in our laboratory. This study aims to determine how stimulus load modulates recognition processing of digits.

METHODS

ERPs were recorded from 117 healthy right-handed subjects during a delayed matching-to-sample paradigm, using number stimuli that were either low load (three digits long) or high load (five digits long). The bootstrap method [R. Srebro, A bootstrap method to compare the shapes of two scalp fields, Electroenceph. Clin. Neurophysiol. 100 (1996) 25-32.] was employed to evaluate the topographic features of the VMP revealed in the current source density (CSD) maps.

RESULTS

Response times were significantly shorter for matching stimuli than for non-matching stimuli only for low loads; longer response times were related to higher loads compared to low loads only for matching stimuli. The high loads were related to larger ERP responses. The ERP was significantly smaller for matching than for non-matching three-digit numbers, but not for five-digit numbers. The ERP was also reduced to the test stimuli compared to sample stimuli regardless of stimulus load. Both temporal and frontal regions were involved in the recognition of the digit stimuli, and the left hemisphere was more active in the non-matching processing of digits.

CONCLUSIONS

The VMP spatial pattern in addition to its amplitude is sensitive to stimulus load in the encoding process.

Processing of delta-endotoxin of Bacillus thuringiensis subsp. kurstaki HD-1 in Heliothis armigera midgut juice and the effects of protease inhibitors

Bombyx mori was found to be more sensitive to the protoxins of HD-1 than Heliothis armigera. SDS-PAGE analysis showed that a large amount of activated toxin was yielded from protoxin by B. mori gut juice while little was yielded by H. armigera. Further degradation of activated toxin was observed in H. armigera midgut juice detected by SDS-PAGE. pH influenced the proteolytic activity of the midgut juice significantly, but there was no obvious effect of pH on the degradation of activated toxin. Specific inhibitor study revealed the presence of trypsin, chymotrypsin, and elastase in the midgut juice. TLCK, TPCK, elastatinal and some general serine protease inhibitors successfully prevented the excessive degradation of protoxin in H. armigera midgut juice. Chymotrypsin inhibitors showed strong inhibitory effects against the further degradation of activated toxin, indicating that chymotrypsin played a major role in the process. It was presumed that the excessive degradation of protoxin in H. armigera midgut juice was responsible for the low sensitivity of the insect to Bt. Further study demonstrated that the excessive degradation in vitro was triggered by SDS treatment. However, all of the tested serine protease inhibitors expressed synergism with protoxin against H. armigera larvae, suggesting that the excessive degradation of protoxin may occur in vivo to some extent and may be triggered by receptor binding of activated toxin.

ERP components in category matching tasks

The current experiment attempts to investigate (1) the effect of semantic information on the ERP correlate of visual short-term memory (VMP) and (2) the utilizing of the ERP as an objective investigative tool for the clinical observation of the existence of category-specific brain systems. Event-related potentials (ERPs) were recorded from 61 locations on the scalp of 39 healthy adults in a category (either animals or fruits/vegetables) match/non-match S1-S2 paradigm. The ERPs revealed a substantially smaller amplitude for the category matching than for non-matching pictures at the posterior brain regions, with greater activation of temporo-occipital brain regions changing from the right hemisphere at first to the left hemisphere later, as demonstrated by the current source density (CSD) maps. The ERPs elicited by the category of animal were larger than the vegetable category, similarly, the animal-animal matching condition elicited larger ERPs than did the vegetable-vegetable matching condition. In addition, the topographic distribution of the vegetable-elicited ERPs revealed additional involvement of the right frontal cortex which was absent in the topographic distribution of the animal-elicited ERPs. The spatial pattern of the VMP possesses features specific to semantic processing, and the ERPs differentiate the animal category from the vegetable category, suggesting an objective on-line method to investigate the category-specific information processing among brain-damaged patients.

Nitric oxide selectively inhibits intracellular Ca++ release elicited by inositol trisphosphate but not caffeine in rat vascular smooth muscle

The present study was designed to investigate whether nitric oxide (NO) could interfere with intracellular Ca++ release through different pathways in vascular smooth muscle. Phasic contractions of rat aorta induced by phenylephrine or caffeine in Ca++-free solution were used as an indicator of intracellular Ca++ release through the inositol 1,4,5-triphosphate receptor pathway and the ryanodine receptor pathway, respectively. In addition, cytoplasmic Ca++ concentration ([Ca++]i) in vascular smooth muscle cells was determined by fluorescence measurement. Acetylcholine (ACh) inhibited the phenylephrine-evoked phasic contractions in Ca++-free solution in endothelium-intact but not -denuded aortic rings in a dose-dependent manner. However, ACh did not affect the action of caffeine. The inhibition by ACh was blocked completely by the NO synthase inhibitor Nomega-nitro-L-arginine, which could be reversed totally by L-arginine but not D-arginine. Methylene blue, a soluble guanylate cyclase inhibitor, also abolished the inhibition by ACh. Sodium nitroprusside, an NO donor, attenuated the phenylephrine- but not caffeine-induced phasic contractions in denuded aortic rings in Ca++-free solution. The effect of sodium nitroprusside was reversed substantially by methylene blue. Furthermore, sodium nitroprusside inhibited the elevation of [Ca++]i induced by phenylephrine in vascular smooth muscle cells isolated from rat aorta in the absence of extracellular Ca++, which could be abolished significantly by methylene blue. These results suggest that NO selectively inhibits intracellular Ca++ release stimulated by inositol 1,4,5-triphosphate, but not caffeine in vascular smooth muscle.

Vasopressin-induced activation of protein kinase C in renal epithelial cells

Recent studies indicate that the actions of arginine vasopressin (AVP) and other agonists that stimulate electrogenic sodium transport in renal epithelial A6 cells are linked to a Ca(2+)-mobilizing signal transduction mechanism that involves generation of inositol trisphosphate. Since diacylglycerol is the other product in this pathway, studies were performed to determine the possible role of PKC in the stimulation of sodium transport. AVP induced a biphasic increase in diacylglycerol generation, characterized by an initial rapid rise and then a sustained elevation, and PKC activation, reflected by phosphorylation of a specific 80 kDa myristoylated alanine-rich PKC substrate (MARCKS). To determine the PKC isoform(s) involved in this process, immunoblot analysis was performed using antisera that recognize both classical PKC isoforms, XPKC-I and XPCK-II, cloned from Xenopus oocytes. The transcripts of both isoforms were expressed in the A6 cell. Since protein recognized by antisera was translocated from cytosol to the particulate fraction after exposure to AVP, one or both isoforms were activated in the A6 cell. Further studies showed that cyclohexyladenosine and insulin, additional agonists of sodium transport in A6 cells, also stimulated phosphorylation of MARCKS. These results argue that Ca(2+)-dependent PKC is involved in the action of AVP, and that of other agonists, which stimulate sodium transport.

Structural characterization and topology of the second potential membrane anchor region in the thromboxane A2 synthase amino-terminal domain

Thromboxane A2 synthase (TXAS) has been proposed to have two membrane-bound regions located in the NH2-terminal domain [Ruan, K.-H., Wang, L.-H., Wu, K. K., and Kulmacz, R. J. (1993) J. Biol. Chem, 268, 19483-19489; Ruan, K.-H., Li, P., Kulmacz, J. R., and Wu, K. K. (1994) J. Biol. Chem, 269, 20938-20942]. To test this hypothesis, a solution structure in membrane mimetic environments of a synthetic peptide corresponding to the second region of the NH2-terminal domain (TXAS residues 33-60) has been investigated by circular dichroism (CD), 2D nuclear magnetic resonance (NMR) spectroscopy, and peptidoliposome reconstitution. CD spectroscopy indicated that the peptide adopted a structure with significant alpha-helical content in 30% trifluoroethanol (TFE) or in dodecylphosphocholine (DPC) micelles, which mimic hydrophobic membrane environment. Through a combination of 2D NMR experiments in the presence of TFE or DPC micelles, complete 1H NMR assignments of the peptide have been obtained and the structure of the peptide has been determined. NH2-terminal segment of the peptide takes on a well-defined alpha-helical conformation; the center segment of the peptide, containing three prolines, adopts a bent conformation, and the C-terminal segment of the peptide exists in a mixture of rapidly interconverting conformations. These results provide direct structural evidence that residues 33-60 of the TXAS NH2-terminal domain contain a second membrane anchor region, with at least residues 35-46 having their helical structure expected for hydrophobic interaction with the membrane. The orientation of the peptide in DPC micelles was evaluated from the effect of incorporation of a spin-label 12-doxylstearate into the micelles. The peptide portions, found to be immersed in the micelles, include the helical segment, the bent segment, and some hydrophobic residues within the C-terminal segment. Two additional synthetic peptides, one corresponding to the NH2-terminal helical segment (TXAS residues 33-46) and the other including the bent and the C-terminal segments (TXAS residues 47-60) were analyzed for their ability to incorporate into peptidoliposomes. The helical peptide readily incorporated into liposomes; the other peptide did not. These results support the presence of a second functional membrane anchor region localized to the helical segment within TXAS residues 33-46, with passive membrane contacts in the bent and the C-terminal segments of the peptide (TXAS residues 47-60) due to immersion of the helical in the membrane.

New perspectives in mental health services in Shanghai

This article reports developments in mental health services in Shanghai during the past several years, including family education and involvement, new forms of community services, psychological consultation programs, hot line services and crisis intervention, dealing with psychological problems in general hospitals, and nursing care for elderly patients with dementia. The authors reevaluate and outline the new perspectives in mental health services in Shanghai.

[Dual-wavelength method for analysis of overlapped high performance liquid chromatographic peaks with multiwavelength diode array detection]

High performance liquid chromatography (HPLC) is one of the analytical techniques most frequently used. Its chief limitation lies in the need to avoid the overlap of chromatographic peaks as much as possible. In this work, a new method for the treatment of HPLC data allowing the resolution of extensively overlapped peaks has been developed. This method uses a diode array spectrophotometer and treats the generated data by dual-wavelength method. Its foundation is based on calculating the peak areas for two given wavelength in order to determine the contribution of each component to the mixed peak. Because the area is the integration of absorption at different wavelength, the wavelength can be chosen from the absorption spectra of the two compounds for eliminating the interference of the other component in the mixture, and the concentration of the corresponding component can be calculated. This method provides a simplification of the calculation process and a reduction of the amount of the data used. The applicability of the method is checked on mixture of two aromatic nitro-compounds. Though the two components almost have the same retention time and their peaks overlapped extensively, the results are satisfactory.

[An observation on calcium ion overloaded injury and the effect of calcium channel blocker in rabbit retina under acute ocular hypertension]

OBJECTIVE

In order to observe the retinal injury by Ca+2 overload under ocular hypertension and the effect of verapamil treatment.

METHODS

Atomic absorption spectrophotometry and phosphate pyroantemonate (PPA) means were used to observe the changes of rabbit retinal Ca+2 content and the therapeutic effects of verapamil under this situation.

RESULTS

In the ocular hypertensive group, Ca+2 increased from the moment of intraocular pressure elevation, and with prolongation of time, it tended to increase steadily. In comparison with the control group, there were significant differences (P < 0.01). PPA showed that the longer the time, the more obvious swollen the mitochondria and much more calcium deposition in the mitochondria. In the group of rabbits with ocular hypertension having taken verapamil treatment, the Ca+2 content was markedly lower, the changes of mitochondria milder and the Ca+2 deposition in mitochondria less than that of the ocular hypertension group without treatment.

CONCLUSION

Calcium-overload may be one of the reasons of retinal injury under ocular hypertension, while calcium-blocker may alleviate this injury.

In vitro expansion of GGC:GCC repeats: identification of the preferred strand of expansion

The human fragile-X syndrome, a major cause of inherited mental retardation, is associated with expansion of the trinucleotide repeat GGC:GCC. Repetitive sequences in DNA are subject to slippage during catalysis by DNA polymerases. We characterized the extent of slippage of synthetic GGC:GCC repeats by various DNA polymerases: Taq DNA polymerase, Klenow fragment of DNA polymerase I, DNA Sequence, DNA polymerase-alpha and polymerase-beta, as well as HIV reverse transcriptase. All of these enzymes were found to expand GGC:GCC repeats, with the most extensive expansion exhibited by Taq DNA polymerase. Starting with a template and primer, each 15 nucleotides (nt) in length, the product of one round of synthesis by Taq polymerase is as long as 250 nt. Sequence analysis of cloned DNA fragments expanded by Taq polymerase indicates that expansion involves multiple triplet additions and that it is asymmetric. The asymmetric distribution of terminal nucleotides in the expanded product is consistent with active expansion of the GCC strand and passive additions onto the GGC strand. The preferential elongation and expansion of the GCC strand was confirmed in studies utilizing longer repeats within a single-stranded M-13 template.

[Superoxide dismutase and malonyl dialdehyde in human pulp tissue]

A total of 21 pulps were collected from 12 inflamed and 9 normal cases. SOD activity and MDA content were identified in the normal and inflamed pulpal tissues. In the inflamed pulpal tissues, SOD activity and MDA content were significantly increased than those in the normal tissues. The results demonstrated that the inflammation of pulpal tissues resulted in the increasing of the reactivity of superoxide radical and lipid peroxide (LPO). The results also indicated that human dental pulp possessed an endogenous defense mechanism to protect the tissue components from the toxic effects of the reactive oxygen intermediates.

Solution structure of a two-base DNA bulge complexed with an enediyne cleaving analog

Nucleic acid bulges have been implicated in a number of biological processes and are specific cleavage targets for the enediyne antitumor antibiotic neocarzinostatin chromophore in a base-catalyzed, radical-mediated reaction. The solution structure of the complex between an analog of the bulge-specific cleaving species and an oligodeoxynucleotide containing a two-base bulge was elucidated by nuclear magnetic resonance. An unusual binding mode involves major groove recognition by the drug carbohydrate unit and tight fitting of the wedge-shaped drug in the triangular prism pocket formed by the two looped-out bulge bases and the neighboring base pairs. The two drug rings mimic helical DNA bases, complementing the bent DNA structure. The putative abstracting drug radical is 2.2 +/- 0.1 angstroms from the pro-S H5' of the target bulge nucleotide. This structure clarifies the mechanism of bulge recognition and cleavage by a drug and provides insight into the design of bulge-specific nucleic acid binding molecules.

Solution structure of an antiparallel purine motif triplex containing a T.CG pyrimidine base triple

BACKGROUND

Triplex formation is an approach of potential use in regulating and mapping of gene sequences. However, such applications have been limited to homogeneous sequences consisting of stretches of purines or pyrimidines. Understanding how heterogeneous duplexes are recognized by a third strand oligonucleotide at the atomic resolution level is an essential step toward broadening the application of triplex formation into biochemical and biomedical areas.

RESULTS

The solution structure of an antiparallel triplex (RRY6) containing a site of inversion (i.e. a T within a homopurine stretch, forming a T.CG base triple) has been determined using NMR-restrained computations in the presence of explicit water. The results reveal that within the RRY6 triplex the conformation of the duplex is mostly B-like and that of the third strand exhibits significant variations in interbase separations and backbone torsion angles. A major displacement of the inversion site T sugar in a 5'-direction, accompanied by the tilt of the T base in T.CG, was observed. The T.CG base triple contains a single hydrogen bond between T O4 and the exposed C amino proton and is stabilized by a number of interstrand and sequential van der Waal contacts. The structural comparisons of RRY6 with two related triplexes indicate localized perturbation at the non-classical base triple site. Various triplexes contain sugars in the C2'-endo family and the global features of their duplexes are similar.

CONCLUSIONS

This study provides valuable information concerning the molecular basis of the specific recognition of a Watson-Crick base paired C residue at the inversion sites in the antiparallel triplex and should lead to general rules for designing triplexes containing heterogeneous sequences.

Annexin II up-regulates cellular levels of p11 protein by a post-translational mechanisms

Annexin II (p36) and p11, which belong to two different families of calcium-binding proteins, are able to form a heterotetrameric protein complex (p36)2(p11)2 called calpactin I. As these proteins were detectable only in the presence of each other in a variety of cell lines, we studied the mechanisms of regulation of cellular levels of annexin II and p11. In cells expressing p11 messenger RNA, p11 protein is undetectable unless annexin II is also expressed. As an example, the hepatoblastoma HepG2 cell line displays no detectable annexin II nor p11 protein, although it expresses p11 mRNA. The overexpression of annexin II by gene transfer into HepG2 cells leads to the up-regulation of the cellular levels of p11 by a post-translational mechanism. In the presence of annexin II, there is no major change in the p11 transcript levels, but the half-life of the p11 protein is increased more than 6-fold. Thus, the degree of expression of annexin II, which varies according to different states of cellular differentiation and transformation, is an essential factor in the regulation of cellular levels of p11.

High-resolution SSCP analysis using polyacrylamide agarose composite gel and a background-free silver staining method

We describe here an improved methodology for single-strand conformation polymorphism (SSCP) analysis of PCR products. The method utilizes a polyacrylamide agarose composite gel and background-free silver staining. In comparison with conventional pure polyacrylamide gel for PCR-SSCP analysis, the composite gels have a much greater mechanical strength and improved resolution. The background staining commonly seen in many silver staining protocols has been eliminated by incorporation of thiosulfate, which can prevent nonspecific deposits of silver salts. As shown in our titration tests, the composite gel and background-free silver staining together have allowed clear identification of point mutations in samples containing as little as 5% of the target sequences, the same sensitivity achieved by radioactive labeling methods.