The role of peroxisomes in oxidative stress

Abstract

Peroxisomes are one of the main sites in the cell where oxygen free radicals are both generated and scavenged. The balance between these two processes is believed to be of great importance for proper functioning of cells and has been implicated in aging and carcinogenesis. To gain more insight in the functioning of the peroxisome, we developed a new method to measure the biophysical properties of these organelles in the living cell. To achieve this we made use of small fluorescently labelled polypeptides with a peroxisomal targeting signal of type I. Using such a probe, we determined the peroxisomal matrix is alkaline and that the peroxisomal membrane is impermeable to protons under physiological conditions. Oxidative stress deriving from peroxisomal b-oxidation is to be detoxified by a set of anti-oxidant enzymes. In this thesis I show that next to the well known enzymes catalase and MnSOD, the sterol carrier protein 2 (SCP2) plays a role in the protection against oxidative stress. We show that mice lacking this protein have higher basal levels of lipid-peroxidation in the liver and develop liver tumours. In a model I propose that this peroxisomal protein shields poly-unsaturated fatty acyl-CoA esters from free radicals that are present in the organellar matrix. The regulation of anti-oxidant activity in the cell has been a matter of study. We show that MnSOD is directly regulated by the daf-16 like Forkhead transcription factors. Forkhead transcription factors are negatively regulated by phosphatidyl-inositol-3-OH-kinase (PI-3-kinase) through phosphokinase B (PKB). Catalase and SCP2 are also increased upon activation of the Forkheads, but until now we do not know whether this reflects a direct regulation. A model is proposed for the regulation of anti-oxidant capacity of the cell in relation to metabolism.