Chemical reduction of 3-oxo and unsaturated groups in fatty acids of diphosphoryl lipid A from the lipopolysaccharide of Rhodopseudomonas sphaeroides: Comparison of biological properties before and after reduction

Unlike the diphosphoryl lipid A (DPLA) derived from toxic lipopolysaccharide of Escherichia coli and Salmonella strains, the DPLA from nontoxic lipopolysaccharide of Rhodopseudomonas sphaeroides ATCC 17023 is biologically inactive. This could be due to the presence of 3-oxotetradecanoic and Δ⁷-tetradecenoic acids. These two fatty acids in R. sphaeroides DPLA were catalytically reduced in platinum oxide/H₂ to the 3-hydroxy and saturated fatty acids, respectively. The biologically active E. coli DPLA was also treated with platinum oxide/H₂, but as expected, the reduction step did not change the structure. These two preparations were then compared with the untreated samples for biological activity in three select in vitro assays. Over a range of 0.01-100 ng/ml, both normal and reduced DPLA from R. sphaeroides were inactive in priming phorbol myristate acetate-stimulated superoxide anion release in human alveolar macrophages. Over a range of 10-10³ ng/ml, both samples failed to induce tumor necrosis factor in the RAW 264.7 murine macrophage cell line. The reduced DPLA marginally activated 70Z/3 pre-B cells at concentrations of 0.1-30 μg/ml. In every case, both normal and platinum oxide/H₂-treated E. coli DPLA were biologically active. These results indicate that the lack of biological activity of R. sphaeroides DPLA is not due to the presence of 3-oxo and unsaturated fatty acids, but rather to one or more of the following: (i) presence of only five fatty acyl groups (compared to six in active lipid A); (ii) presence of 3-hydroxydecanoic acids (rather than 3-hydroxytetradecanoic, in active lipid A); (iii) greater variation in size of the fatty acids.