IDENTIFICATION OF HUMAN LIVER CARBOXYLESTERASE AS ONE OF THE PROTEINSINVOLVED IN PLASMODIUM-FALCIPARUM MALARIA SPOROZOITE INVASION IN PRIMARY CULTURES OF HUMAN HEPATOCYTES

Background/Aims: In a previous study, me have demonstrated that primar y human hepatocytes in culture are susceptible for Plasmodium falcipar um sporozoite invasion and for development of parasites into exoerythr ocytic forms. In a separate study we demonstrated the involvement of t wo human liver plasma membrane proteins (55 kD and 20 kD) in the invas ion of P. falciparum sporozoites in vitro. In this study, we have unra velled the nature of the 55 kD protein. Methods: For the identificatio n of this protein, a 53-58 kD membrane protein fraction from human liv er was isolated, radioactively labelled, incubated with sporozoites an d cross-linked, After reduction of the cross-linker, the released prot eins were mixed with unlabelled 53-58 kD protein fraction and separate d on two-dimensional SDS-PAGE, Autoradiography showed a single spot co rresponding to a protein of 55 kD and pI of 5.7-5.8. Results: Amino ac id sequencing revealed the 55 kD protein as carboxylesterase, The biol ogical activity of purified human liver carboxylesterase and of an ant i-serum against carboxylesterase on sporozoite invasion in vitro was e valuated, Human carboxylesterase as well as a rabbit antiserum against carboxylesterase inhibited the invasion of P. falciparum sporozoites into primary human hepatocytes in culture, A number of carboxylesteras e cDNA clones were isolated from a human liver cDNA library, Sequence analysis revealed two different iso-types, Immunoaffinity purified rec ombinant human carboxylesterase was shown also to inhibit the invasion of sporozoites into primary human hepatocytes, Immunocytochemical ana lysis of the localisation of carboxylesterase in primary cultures of h uman hepatocytes using specific antibodies, showed its presence inside the hepatocytes and on the membrane. Conclusions: Carboxylesterase pl ays a role in the invasion process of P. falciparum sporozoites into h uman hepatocytes in vitro. The implications of these findings are furt her discussed.