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By using single-strand conformation polymorphism (SSCP) analysis of three amplicons of the cytochrome b gene obtained by the polymerase chain reaction (PCR) it was possible to differentiate between various species of tunas and bonitos processed as canned fish. Four different techniques were used to produce single-strand DNA (ssDNA): (i) Denaturation of double-strand DNA (dsDNA) by formamide and alkali, (ii) two-step asymmetrical PCR, (iii) one-step asymmetrical PCR, and (iv) exonuclease digestion of the phosphorylated strand of dsDNA. The technique rendering optimal results depended on the type of amplicon (i.e. the sequence).

An enzyme‐specific staining method for trimethylamine oxide demethylase (TMAO‐ase) was developed. Direct visualization could be reached by coupling the reactions of the specific TMAO‐ase assay with another reaction step generating as final product a dark‐blue formazan. For these purposes 3‐(4,5‐dimethyl‐2‐thiazolyl)‐2,5‐diphenyl‐2H‐tetrazolium bromide (MTT) as tetrazolium salt and phenazine methosulfate (PMS) as electron transfer substance were used. Clear, dark‐blue colored bands could be detected on 300 μm isoelectric focusing gels (IEF). Comparisons of enzyme‐stained and protein‐stained gels showed that diffusion could not be observed and that the band pattern of TMAO‐ase could also be seen in the protein stain. The pI range where TMAO‐ase was located was 5.6—6.6 for extracts and 6.2—6.6 for partially purified TMAO‐ase. Specificity of stained TMAO‐ase bands was assessed by the preparation of staining solution without the substrate trimethylamine oxide (TMAO) and by extraction of TMAO‐ase from the gel and performance of the specific TMAO‐ase assay.