[The contribution of peroxisomes to lipid metabolism]

Peroxisomes are ubiquitous subcellular organelles. They contain catalase and hydrogen peroxide-producing oxidases like fatty acyl-CoA oxidase. The latter enzyme is part of a special fatty acid beta-oxidation system which shortens long-chain fatty acids. The middle-chain acids formed are subsequently degraded by mitochondria. The capacity to remove very long fatty acids and trans-unsaturated acids found in hydrogenated oils is restricted to peroxisomes. Essentially, the peroxisomal beta-oxidation system is not constitutive but inducible by certain hypolipidaemic compounds which are distinguished by their capacity to lead to proliferation of peroxisomes. Thyroid hormones as well as prolonged exposure to cold and high fat diets, esp. with long-chain unsaturated fatty acids, also induce beta-oxidation and peroxisome proliferation. Two other beta-oxidative reactions namely the removal of the cholesterol side-chain, leading to the formation of bile acids, and the degradation of dicarboxylic acids as formed by omega-oxidation of fatty acids were shown to be connected with peroxisomes. Presumably also 3-hydroxy-3-methyl-glutaryl-CoA reductase, the key enzyme of cholesterol biosynthesis exists in a peroxisomal moiety. NADPH consumed in this reaction (and in the dihydroxyacetone phosphate pathway of glycerolipid synthesis) might be provided by glucose-6-phosphate dehydrogenase which was recently also found in peroxisomes. Peroxisomes are indispensable in forming saturated ether lipids and plasmalogens because alkyldihydroxyacetone phosphate synthase is a membrane enzyme exclusively located in peroxisomes. Certain other enzymes of the dihydroxyacetone phosphate pathway of glycerolipid synthesis are also found in peroxisomes. Because of the combination of oxidases like fatty acyl-CoA oxidase and catalase and the feasibility of reoxidising NADH within the peroxisomes the aerobic metabolism of peroxisomes is energy-wasting. Therefore they might be important in chemical thermogenesis and in the control of body weight. For all these reasons peroxisomes must be essential for human metabolism. This is further demonstrated by genetically caused disorders: Total absence of peroxisomes is connected with the fatal cerebro-hepatorenal Zellweger syndrome. Defective peroxisomal beta-oxidation is manifested in Schilder's disease (adrenoleukodystrophy) characterized by accumulation of very long fatty acids. Peroxisomes perform a number of complementary and auxiliary reactions in general cell metabolism, in particular the cata- and anabolism of certain lipids, and therefore deserve consideration in clinical chemistry.