Enzymes of plasmalogen biosynthesis in microperoxisomes of guinea-pig intestinal mucosa

Abstract

BIOCHEMICAL SOCIETY TRANSACTIONS electron microscopical observations [ 3 I . for biochemical analysis of enzyme activity and some liver sections were stained with gallocyanin stain for image analysis using the Quantimet QU20 to measure the nuclear area and number of hepatocytcs (S. C. Price &L R. S. Ahmed. unpublished work).DEHP is known to cause hepatomegaly and induce both peroxisomal enzymes and a specific form of cytochrome 1'-450 [ 21. In these experiments, biochemical assays showed an increase in liver size, in peroxisomal enzymes ( Fig. 1 ) and, to a lesser extent, in total cytochrome 1'-450 (not illustrated) during first feeding and on subsequent treatment with the DEHP-containing diet. After reversal, liver size and peroxisomal enzymes decreased, but not to the control values.Electron microscopy clearly showed peroxisome proliferation and, t o a lesser extent, proliferation in mitochondria and smooth endoplasmic reticulum and reduction of rough endoplasmic reticulum in the livers of rats receiving the DEHP-containing diet at the time of death. In groups of animals treated with DEHP and then returned to control diet there remained some difference in numbers of peroxisomes and mitochondria between test and control groups. A more marked difference. however. was that in animals prcviously treated with DEHP the rough endoplasmic reticulum was reorganized into very tight stacks with reduced cisternal space.Image analysis of nuclear number and size showed the expected changes in cell size and ploidy with increasing age 131 and also that the increase in liver size was due to an increase in cell number, not in cell size. This is consistent with earlier results showing that treatment with DEHP leads first to hyperplasia, then to hypertrophy 141. The mean nuclear area was increased in animals actually receiving DEHP (Fig. 1 ) and examination o f the frequency distribution showed that this was due to an increase in tetraploid nuclei. On withdrawal of DEHP, both nuclear number and nuclear size returned towards control values.The results obtained in these experiments indicate first that if rats are treated with DEHP, then returned t o control diet, and then re-fed DEHP. liver size increases more rapidly than in rats never exposed to the compound. Secondly, the increase in cell number on treatment with DEHP is accompanied by an increase in ploidy. Thirdly, o n reversal, the reduction in liver weight is due to a fall in cell number and accompanied by a reduction in ploidy. While reductions in cell number have been observed in other tissues, such as the thyroid gland following withdrawal of chemical stimuli 151, it is generally thought that an increase in ploidy is irreversible so that the reduction in ploidy can only bc explained by selective loss of tetraploid cells or by a replacement of part of the population of liver cells by a new population.

The Biosynthesis and Functions of plasmalogens.

“…This by-passes the peroxisome. Studies carried out by Gitsham et al [17] support this view. Mucosal cells from small intestine of guinea pigs were examined to determine where the first two enzymes in the biosynthesis of plasmalogens were situated.…”

“…It is interesting to note, at this point, that hypolipidemic drugs affect total plasmalogen content in the intestine of guinea pigs. A threefold increase was noted after short-term administration of the drug [17]. This rise in plasmalogen content may increase the amount incorporated into lipoproteins and in turn decrease the abnormal accumulation of cholesterol by macrophage cells.…”

Plasmalogens the neglected regulatory and scavenging lipid species

“…1B. The first two steps of plasmalogen biosynthesis take place exclusively in peroxisomes (Gitsham et al, 1989;Hardeman and van den Bosch, 1989;Singh et al, 1993) and all the involved enzymes have specific topological localizations within the organelle (Brites et al, 2004a). Synthesis begins with the esterification of dihydroxyacetone phosphate (DHAP) with a long-chain acyl-CoA ester by the enzyme DHAP-acyltransferase (DHAP-AT) (Hajra, 1997).…”

Proteins and enzymes of the peroxisomal membrane in mammals

“…Although microsomal glycerol-3-phosphate acyltransferase (G3P-AT) has a dual catalytic activity (G3P-AT and DHAP-AT activities) [119,120], the main function of the acyI-DHAP pathway lies in the synthesis of ether lipids, which have acyI-DHAP as an obligate precursor. Indeed, cellular DHAP-AT activity has been shown to be almost exclusively localized in the peroxisome fraction of different cells and tissues, including guinea pig [11 I, 121 -123], rat [72,111,124,125] and mouse [126] liver, rat brain [101,111,127], rat [126] and mouse [128,129] kidney, guinea pig intestinal mucosa [130,131], human skin fibroblasts [94], preadipocytes 3T3-L 1 [132], and hepatoblastoma ceils HepG2 [133]. The enzyme G3P-AT is virtually absent from peroxisomes [111].…”

“…Although the product (acyI-DHAP) of the reaction catalyzed by DHAP-AT is transported very fast across the membrane and can diffuse to the endoplasmic reticulum, it is also utilized at a high efficiency as substrate by alkyl-DHAP synthase, suggesting a close interaction of the two enzymes in the peroxisomal membrane [115,116]. Indeed, aikyI-DHAP synthase has generally been found to be colocalized with DHAP-AT in peroxisome fractions on centrifugation [77,101,111,123,130,132,142,150,151]. As has been observed for DHAP-AT, lack of peroxisomes in some inherited diseases also leads to a deficiency in alkyI-DHAP synthase activity [135,[152][153][154][155].…”