Peroxisomal and mitochondrial enzymes involved in lipid metabolism : studies on function and regulation

Fatty acids constitute a major part of the energy that we obtain from the diet and are
also the principal source for mammals to store energy. To use the incoming or stored
fatty acids as energy, the fatty acids needs to be metabolized of which the majority of
fatty acids will be degraded by the mitochondrial β-oxidation system that in the end
generates energy to the cell in the form of ATP. However, this organelle is not able to
handle all kinds of fatty acids of which very long chain fatty acids, long chain methylbranched
fatty acids and dicarboxylic acids are such cumbersome fatty acids. Therefore
a second organelle, the peroxisome, is required for metabolism of these particular fatty
acids. Also peroxisomes contain a β-oxidation system and similar to the mitochondrial
system is the initial substrate a CoA-esterified fatty acid, so-called acyl-CoA.
This thesis will focus on some enzymes that are active on these acyl-CoA esters, but
that are not directly involved in the β-oxidation per se. Instead they contribute to the
regulation of both acyl-CoA and free coenzyme A levels in different cellular
compartments. This thesis will also include how these fatty acid degrading systems can
be regulated at gene level by affecting different transcription factors by dietary ligands
and by fasting.



The peroxisomal Nudix hydrolase 7α (NUD7α), previously believed to be a CoASH
degrading enzyme, was demonstrated to be a medium chain diphosphatase, most active
on medium chain acyl-CoA esters, to produce 3’,5’-ADP and the corresponding 4’-
acylphosphopantetheine thereof. NUDT7α expression and activity was down regulated
by PPARα activation, which would prevent CoASH degradation and support a high
rate of the β-oxidation in peroxisomes during these conditions.



Peroxisomes are not only needed for the degradation of complex lipids, but are also
essential for many other metabolic pathways such as bile acid and etherphospholipid
synthesis and the degradation of D-amino acids and glyoxylate. The expression of gene
transcripts that code for the proteins involved in these peroxisomal pathways was
investigated almost throughout the whole mouse body with the aim to map the tissue
expression of these pathways. The peroxisomal β-oxidation system is present in all
examined tissues, however with differences in magnitude. More specifically expressed
pathways are e.g. glyoxylate and D-amino acid degradation pathways. Auxiliary
enzymes to the peroxisomal β-oxidation showed tissue specific expression, suggesting
a high degree of tissue specific metabolite patterns, also being dependent on the
metabolic state. The study also shows that PPARα is of major importance for the
regulation in liver of the peroxisomal “transcriptome” during fasting.



Mitochondria degrade both fatty acids and amino acids and the mitochondrial acyl-
CoA thioesterase 9 (ACOT9) was shown to hydrolyze both long chain acyl-CoAs as
well as short chain acyl-CoA intermediates and products of branched-chain amino acid
metabolism. Kinetic characterization of the enzyme suggests a thigh regulation of the
activity during different metabolic conditions in the mitochondria.



Dietary ω-3 PUFAs from fish oil (FO) and krill oil (KO) cause different changes in
lipid profiles and gene regulation when supplemented to mice. FO lowered most
plasma lipids whereas KO only significantly lowered non-esterified fatty acids in
plasma. FO showed a classical PPARα activation response by up regulating genes for
fatty acid utilization and oxidation whereas KO down regulates genes for cholesterol
and fatty acid synthesis