Lactoferrin Interaction with Actinobacillus actinomycetemcomitans

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Abstract
The interaction of lactoferrin with Actinobacillus actinomycetemcomitans was examined in a 125I‐labeled protein binding assay. The binding of human and bovine lactoferrins reached maximum within 1 h. Lactoferrin binding to the bacterium was pH‐dependent and reversible. Scatchard analysis indicated the existence of two different types of binding sites on the bacterium, one with a high affinity constant kα=8.8×10−7 M) and the other with a low one (kα=1.8×10−6 M). Bacteria in the exponential phase of growth showed higher binding than cells in the stationary phase. Bacteria grown in medium containing serum and/or lysed erythrocytes bound lactoferrin to a lesser extent. Heat‐inactivated serum, lysed erythrocytes and other proteins such as mucin and laminin inhibited lactoferrin binding to A. actinomycetemcomitans in a competitive binding assay. Sodium dodecyl sulfate polyacrylamidegel electrophoresis and Western blot analysis of the cell envelope as well as the outer membrane of A. actinomycetemcomitans revealed lactoferrin‐reactive protein bands at 29 kDa and 16.5 kDa. The 29‐kDa band displayed a heat‐modifiable lactoferrin‐reactive form with a molecular weight of 34 kDa. Neither proteinase K‐treated cell envelope nor lipopolysaccharide of this bacterium showed reactivity with lactoferrin. These data suggests a specific interaction of lactoferrin with outer membrane proteins of A. actinomycetemcomitans.