Lipid A fractions analyzed by a technique involving thin‐layer chromatography and enzyme‐linked immunosorbent assay

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Abstract
A modified enzyme‐linked immunosorbent assay (ELISA), with alkaline phosphatase as enzyme, was used for the study of antigenicity of lipid A fractions directly on thin‐layer chromatographic (TLC) plates. For visualization a gel slab containing the enzyme substrate was placed on the plate containing enzyme‐conjugated antibodies. The plate was read by a thin‐layer chromatogram spectrophotometer. The immunoassay was both highly specific and quite sensitive. Sensitivity was superior to levels obtained by staining the plate with molybdenum blue (for phosphate) or orcinol (for carbohydrate). Fractions of lipid A from Escherichia coli 0111, Shigella flexneri or Salmonella minnesota R595, after being separated by thin‐layer chromatography, were analyzed using rabbit anti‐(lipid A) serum. Patterns obtained by scanning the same plates for phosphate staining and for the TLC‐ELISA corresponded well. For comparison with TLC‐ELISA, an inhibition assay was run using a tube ELISA. The tube ELISA, run in aqueous medium, showed that fractions 6–8 (those having the highest RF values) had the least activities. In contrast, TLC‐ELISA did not detect large differences between fractions 2–7. This discrepancy probably reflected limited aqueous solubility of fractions 6 and 7. We conclude that TLC‐ELISA might reveal antigenic activities of lipids that could be missed by other methods. The data suggested that all fractions, except for fraction 8, were similar in their antigenicity by TLC‐ELISA. Differences in antigenicity between the fractions occurred when the fractions were tested in free form in an aqueous environment and these differences possibly could have been due to different solubilities of individual fractions.