Effect of serum albumin on siderophore-mediated utilization of transferrin iron

Abstract

The effect of serum and serum proteins on enterobactin- and aerobactin-mediated utilization of transferrin Fe was investigated. Serum impeded transfer of Fe from Fe transferrin to enterobactin and from [55Fe]ferric enterobactin to cells of Escherichia coli BN3040 Na1R iuc. Serum had essentially no effect on the rate of these reactions mediated by aerobactin. Three purified serum proteins, human serum albumin, bovine serum albumin and human Ig, were comparable to human serum in their selective ability to interfere with the transfer of 55Fe from [55Fe]ferric enterobactin to E. coli BN3040 Na1R iuc. The inhibitory effect of human serum albumin on the enterobactin-mediated transfer of Fe from [55Fe]transferrin was enhanced by preincubation of the protein with the siderophore. Pretreatment of the bacterial cells with human serum albumin did not affect the rate of utilization of siderophore Fe. A linear, reciprocal relationship was found to hold for human albumin concentration vs. the 1st-order rate constant for the velocity of Fe transfer from Fe transferrin to enterobactin. Binding of serum albumin to enterobactin increased the intensity of the near-UV absorption band of the siderophore and shifted it to longer wavelengths. The stoichiometry of binding to human and bovine serum albumins was established as 1:1; the binding constant for enterobactin and ferric enterobactin was estimated to be in the range 1 .times. 104-1.2 .times. 105 M-1. Evidently, serum albumin acts synergistically with other factors in the serum, such as transferrin, to limit Fe supply and in this way restrict the growth of invading microorganisms.