Probing peroxisornal β-oxidation and the labelling of acetyl-CoA proxies with [1-13C]octanoate and [3-13C] 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-oxidation of long-chain and very-long-chain fatty acids. This conclusion was based on the interpretation of the C-13-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-C-13]octanoate or [3-C-13]octanoate, (i) whether peroxisomal beta-oxidation contributes acetyl groups for malonyl-CoA synthesis, and (ii) the degree of labelling homogeneity of acetyl-CoA proxies (acetyl moiety of citrate, acetate, beta-hydroxy-butyrate, malonyl-CoA and acetylcarnitine). Our data show that (i) octanoate undergoes two cycles of peroxisomal P-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 P-hydroxybutyrate does not reflect the labelling of acetyl-CoA used in the citric acid cycle.