Lipid heptaacyl

Takayama and coworkers (60) introduced the h.p.l.c. separation technique for such amphiphilic molecules as lipid A, and in earlier experiments they applied paired-ion reverse-phase h.p.l.c. for the preparation of homogeneous fractions deriving from 4,-monophosphated lipid A of S. typhimur-ium. The purified preparations obtained were suitable for f.a.b. - m.s. analysis. However, monophosphated lipid A isolated in this way expressed a considerable heterogeneity with respect to the number and location of 0-acyl residues (60). In order to further improve the purification procedure, as well as to obtain lipid A derivatives suitable for n.m.r. spectroscopy, Qureshi et al. (174) prepared the dimethyl phosphate derivative of S. minnesota (R595) lipid A, which, after purification by reverse-phase h.p.l.c. (C18), could be analyzed by1 H-n.m.r. The n.m.r. spectrum of, for example, the heptaacyl lipid A dimethyl monophosphate fraction, unequivocally revealed 0-acyl substitution [14 0(3-OH)J at position 3 and a free hydroxyl group at position 4 of GlcN(I). 

Single crystals of free lipid A or LPS are as yet not available. Therefore, the most promising approach to obtain molecular models is to perform theoretical calculations. After the chemical structures of enterobacterial lipid A had been elucidated, this methodology was successfully applied with heptaacyl S. minnesota lipid A (220) and hexaacyl E. coli Re LPS (221). As an example, Fig. 13 shows the atomic model of the E. coli lipid A molecule, as calculated by Kastowsky et al. (221) using energy-minimization techniques. 

Galanos, C., Luderitz, O., Freudenberg, M., Brade, L., Schade, U., Rietschel, E.T., Kusumoto, S., Shiba, T. Biological activity of synthetic heptaacyl lipid A representing a component of Salmonella minnesota R595 lipid A. Eur J Biochem 160 (1986) 55-59. 

The palmitoyl transferase will utilize lipid X, lipid IVa, Kdo2-lipid IVa (Figure 3), or lipid A as acceptor substrates [50, 51]. Palmitate addition to lipid A is a sub-stoichiometric modification found in E. coli cells treated with ammonium metavanadate [52]. It can occur in addition to the acylations performed by HtrB and MsbB to produce a hepta-acylated lipid A. Other bacteria, such as Salmonella, have very active palmitoyl transferases even in the absence of ammonium metavanadate treatment and a higher proportion of their lipid A contains palmitate [1, 53]. The role of palmitate or heptaacylation in the function of LPS is unknown. 

---