CONSTITUTIVE EXPRESSION OF A SATURABLE TRANSPORT-SYSTEM FOR NONESTERIFIED FATTY-ACIDS IN XENOPUS-LAEVIS OOCYTES

In the presence of 150 mu M BSA, uptake of[H-3]oleate by Xenopus laevis oocytes was a saturable function of the unbound oleate concentration (V-max. 110+/-4 pmol/h per oocyte; K-m 193+/-11 nM unbound oleate). Oleate uptake was three orders of magnitude faster than that of another test substance, [S-35]bromosulphophthalein, and was competitively inhibited by 55 nM unbound palmitate (V-max. 111+/-14 pmol/h per oocyte; K-m 424+/-63 nM unbound oleate) (P < 0.01). Oleate uptake was also inhibited by antibodies to a 43 kDa rat liver plasmamembrane fatty acid-binding protein, a putative transporter of long-chain fatty acids in mammalian cells; uptake of the medium-chain fatty acid [C-14]octanoate was unaffected. Immunofluorescence and immunoblotting demonstrated that the antiserum reacted with a single 43 kDa protein on the oocyte surface. Hence a protein related to the mammalian plasma-membrane fatty acid-binding protein may play a role in saturable uptake of long-chain fatty acids by Xenopus oocytes.