Peroxisome Deficiency Does Not Result in Deficiency of Enzymes involved in Cholesterol Biosynthesis

Abstract

To unravel conflicting literature data on the dependence of cholesterol biosynthesis on functional peroxisomes, we have determined activities and levels of selected cholesterol biosynthetic enzymes in livers of a PEX5 knock-out mouse, a well-characterized animal model for human Zellweger syndrome. All enzymes measured, including putative peroxisomal enzymes, were normally active in the peroxisome-deficient mice indicating that mislocalization of these enzymes to the cytosol does not lead to decreased activity or degradation. Since these findings were in apparent contrast to previously reported data in human livers, we re-examined cholesterol biosynthetic enzymes in livers of Zellweger patients and control subjects. We found that the previously reported deficient enzymes mevalonate kinase and phosphomevalonate kinase are inactivated rapidly in a time-and temperature-dependent manner. This inactivation is not due to degradation as a result of mislocalization since normal levels of protein can be detected. Therefore, the deficiencies of these enzymes in livers of Zellweger patients reflect the bad condition of the livers, rather than mislocalization to the cytosol. Our data explain the conflicting findings in literature and demonstrate that great care should be taken in the interpretation of data obtained in postmortem material (1). To exclude that genetic differences, resulting in different defects in peroxisomal biogenesis, might influence the activity of selected cholesterol biosynthetic enzymes, we also have determined activities and levels of these enzymes in human fibroblasts of patients belonging to various different complementation groups. We included fibroblasts of patients with mutations in genes encoding PEX1, PEX5, PEX7, PEX16, and PEX19, resulting in selective deficiencies in import of PTS1 or PTS2 containing proteins or in a more general defect in peroxisome biogenesis resulting in the complete absence of peroxisomes or peroxisomal ghosts. We measured normal protein levels and activities of enzymes involved in cholesterol biosynthesis in all cell lines. We conclude that peroxisomes are not a prerequisite for cholesterol biosynthesis in humans.