Probing peroxisomal β-oxidation and the labelling of acetyl-CoA proxies with [1-(13)C]octanoate and [3-(13)C]octanoate in the perfused rat liver

We reported previously that a substantial fraction of the acetyl groups used to synthesize malonyl-CoA in rat heart is derived from peroxisomal β-oxidation of long-chain and very-long-chain fatty acids. This conclusion was based on the interpretation of the (13)C-labelling ratio (malonyl-CoA)/(acetyl moiety of citrate) measured in the presence of substrates that label acetyl-CoA in mitochondria only (ratio <1.0) or in both mitochondria and peroxisomes (ratio >1.0). The goals of the present study were to test, in rat livers perfused with [1-(13)C]octanoate or [3-(13)C]octanoate, (i) whether peroxisomal β-oxidation contributes acetyl groups for malonyl-CoA synthesis, and (ii) the degree of labelling homogeneity of acetyl-CoA proxies (acetyl moiety of citrate, acetate, β-hydroxybutyrate, malonyl-CoA and acetylcarnitine). Our data show that (i) octanoate undergoes two cycles of peroxisomal β-oxidation in liver, (ii) acetyl groups formed in peroxisomes contribute to malonyl-CoA synthesis, (iii) the labelling of acetyl-CoA proxies is markedly heterogeneous, and (iv) the labelling of C1+2 of β-hydroxybutyrate does not reflect the labelling of acetyl-CoA used in the citric acid cycle