Excretion of citrate by metacestodes of Mesocestoides vogae

When metacestodes of Mesocestoides vogae were incubated with oxaloacetate, the ¹H NMR spectra of the incubation media established the presence of citrate in addition to the usual worm excretory products succinate, acetate, alanine, and lactate. Given the known impermeability of the inner mitochondrial membrane to multiply charged matrix metabolites, the finding strongly suggests the presence of a transporter to convey citrate from the mitochondria to the cytosol. It is proposed that some of the extramitochondrial citrate serves as a source of cytosolic acetyl-CoA, via the citrate lyase reaction, for conversion to malonyl-CoA and subsequent fatty acid chain elongation. The data demonstrate that because citrate is excreted, further forward steps in the tricarboxylic acid cycle are either very slow or not operating.