Properties and biogenesis of peroxisomes.

Many species of monocellular eukaryots as well as the majority of animal cell and plant tissues show the presence of peroxisomes or microperoxisomes. Their size, shape and internal organization may differ in various cellular types significantly. Typical components of animal cell peroxisomes are the membrane, matrix, low density compartment enriched in lipids, and the compartment containing D-amino acid oxidase. The group of four enzymes (catalase, D-amino acid oxidase, L-alpha-OH-acid oxidase) the location of which had been originally discovered in peroxisomes of hepatocytes of rodents was later widened by approximately forty further enzymes. It is though probable that evolution brought along a reduction and loss of various metabolic functions of peroxisomes and a decrease in the number of enzymes. Peroxisomes are characterized by high variability of the enzymatic content in dependence on the nutritional conditions and the effect of xenobiotics. Fasting, diabetes mellitus, high-lipid diet, peroxisome proliferators induce several peroxisomal enzymes, especially fatty acids beta-oxidation. The mechanism of the impact of heterogeneous substances on the gene transcription has been clarified recently. Substances as fibrates, retinoic acid, polyunsaturated fatty acids activate specific types of receptors-PPAR (peroxisome proliferators activated receptors) belonging to the superfamily of receptors activated by steroid hormones, thyroid hormones, and D-vitamins. A simultaneous induction of several peroxisomal enzymes can be achieved by the linkage between PPAR and specific areas of promotors of particular genes. Such areas-PPREs (peroxisomal proliferator response elements) with five repeated TGA(A/C/T)CT hexanucleotide sequences separated by one nucleotide were discovered in several peroxisomal genes. It is assumed that the stimulation of transcription can be achieved by the linkage between homodimers, and heterodimers of nuclear receptors on these DNA sections. The majority of peroxisomal proteins is synthesised in the cytoplasm, namely on polysomes being in matured forms. Unimpaired biogenesis of peroxisomes requires membrane transport proteins and presence of signal in polypeptide chain of imported proteins (PTS-peroxisomal targeting signal). The function of PTS in many peroxisomal proteins is fulfilled by the C-terminal tripeptide which is composed of amino acids, namely serine, lysine, and leucine (SKL-tripeptide), respectively by a tripeptide with a very similar composition in amino acids. Aside from this signal, still another signal exists, which is located at the N-end of peroxisomal proteins. The role of membrane proteins 70, 35, 256, 22, 15 kDa, is being discussed in relationship to the functions and diseases caused by impaired biogenesis of peroxisomes. (Fig. 4, Ref. 128.)