Growth inhibition of Bacteroides fragilis by hemopexin: proteolytic degradation of hemopexin to overcome heme limitation.
The stimulatory effect of heme on growth of Bacteroides fragilis, an anaerobic human pathogen, was strongly inhibited by hemopexin, an avid (Kd<1 pM) heme-binding plasma protein. Both rabbit and human hemopexins were bacteriostatic for a limited period of time, suggesting an adaptation by B. fragilis to heme-limited growth, and that hemopexin-bound heme can eventually be utilized by the bacteria. The inhibitory effect of hemopexin was lost when heme in the medium was replaced by protoporphyrin IX, which is bound less strongly by hemopexin (Kd approximately 1 microM). Protease activity was detected in the culture supernatant of B. fragilis grown in the presence of heme plus hemopexin but not in the presence of free heme, protoporphyrin IX or protoporphyrin IX plus hemopexin, suggesting that the enzyme(s) is induced by heme macrocycle limitation due to the scavenging effect of hemopexin. This protease activity was able to degrade rabbit hemopexin and human hemopexin, as well as human transferrin and ovalbumin, and may be a due to a serine protease since it was inhibited by phenylmethylsulfonyl fluoride (PMSF) but not by EDTA, leupeptin, pepstatin A or aprotinin. Thus, B. fragilis may overcome hemopexin-mediated heme limitation by secreting inducible protease(s), shown here to make protein-bound heme available to the microorganism.
Rocha
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ER
Department of Microbiology and Immunology, East Carolina University School of Medicine, Greenville 27858-4354, USA. rochae@mail.ecu.edu
Smith
A
A
Smith
C J
CJ
Brock
J H
JH
D000818:Animals; D001441:Bacteroides fragilis / Q000187:drug effects / Q000254:growth & development*; D003470:Culture Media; D004591:Electrophoresis, Polyacrylamide Gel; D010450:Endopeptidases / Q000378:metabolism*; D006418:Heme / Q000494:pharmacology; D006466:Hemopexin / Q000378:metabolism* / Q000494:pharmacology*; D006801:Humans; D011817:Rabbits
D003470:Culture Media; D006418:Heme; D006466:Hemopexin; D010450:Endopeptidases