Biogenesis of peroxisomes and glyoxysomes

Assessment of Possible Carcinogenic Risk to Humans Resulting from Exposure to Di(2-ethylhexyl)phthalate (DEHP)

The purpose of this work was to estimate the degree of risk that might be associated with human exposure to low levels of the plasticizer di(2-ethylhexyl)phthalate (DEHP). DEHP is a common component, sometimes at high concentrations, of polyvinyl chloride (PVC) plastics and was recently reported by the National Toxicology Program (NTP) to be carcinogenic in rats and mice, inducing hepatocellular tumors in both species. This work was also designed to illustrate an approach to risk assessment that attempts to incorporate all available biological data. Based on the dose-response data generated by the NTP bioassays, we have performed extrapolations of risk to low dose levels using several procedures, including some that incorporate inferences from the available data that shed light on the likely relationship between dose level and risk at low dose levels. In drawing these inferences, consideration was given to such factors as genotoxicity, metabolism and pharmacokinetics, and physiological and biochemical effects of DEHP that might reveal its mechanism of action. The relative merits of each of the various risk estimates are described, based on current understanding of DEHP's mode of biological action. It is concluded that DEHP's mechanism of carcinogenicity in rodents most likely involves its ability to induce peroxisome proliferation and related enzymatic changes, although other mechanisms cannot be excluded. If humans and rodents are assumed to be at the same risk at the same daily dose level of DEHP, application of the various low dose extrapolation models leads to the prediction that the daily dose resulting in a lifetime risk of no more than 1 in 1 million would be between 1.5 and 791 mg/kg per day, with the most likely figure being 116 mg/kg per day. If the carcinogenicity of DEHP is dependent upon its pattern of metabolism, however, it would be inappropriate to extrapolate from rodents to man without qualification because of the major quantitative differences in metabolism in rats, mice, and primates, including man. One of the major differences in metabolism of DEHP between rats and mice and primates is in production of a metabolite whose level may be an indicator of the level of peroxisomal activity and, hence, if the peroxisome proliferation theory of DEHP carcinogenicity is correct, of carcinogenic risk. However, the substantial doubt that exists regarding the applicability of rodent carcinogenicity data to humans must be expressed in qualitative terms.

Peroxisomes in intestinal and gallbladder epithelial cells of the stickleback, Gasterosteus aculeatus L. (Teleostei)

The occurrence of microbodies in the epithelial cells of the intestine and gallbladder of the stickleback, Gasterosteus aculeatus L., is described. In the intestine the organelles are predominantly located in the apical and perinuclear zone of the cells and may contain small crystalline cores. In gallbladder epithelial cells the microbodies are distributed randomly. The latter organelles are characterized by the presence of large crystalloids. Cytochemical and biochemical experiments show that catalase and D-amino acid oxidase are main matrix components of the microbodies in both the intestinal and gallbladder epithelia. These organelles therefore are considered peroxisomes. In addition, in intestinal mucosa but not in gallbladder epithelium a low activity of palmitoyl CoA oxidase was detected biochemically. Urate oxidase and L-alpha hydroxy acid oxidase activities could not be demonstrated.

Ontogeny of glyoxysomes in maturing and germinated cotton seeds-a morphometric analysis

Morphometric procedures were used with light and electron microscopy to examine glyoxysome number, volume, shape and distribution as well as mesophyll cell volume, in cotyledons of mature (50 d postanthesis), imbibed (5h) and germinated (24 and 37 h) cotton (Gossypium hirsutum L.) seeds. Additionally, activities of five glyoxysomal marker enzymes in cotyledon extracts were assayed at each of the above ages. Cell volume was determined from photomicrographs of Epon-embedded sections by the point-counting procedure. Analysis of variance showed that cell volume was not different among the tissue segments studied. Glyoxysomes were cytochemically stained for catalase (EC 1.11.1.6) activity with the 3,3'-diaminobenzidine-tetrahydrochloride procedure. Analyses involving both phase and electron microscopy, and two separate sterologic calculations for determining the number of glyoxysomes per cell, indicate that glyoxysomes are numerous in mature seeds, persist through desiccation and imbibition, then increase dramatically in volume (seven fold) but not number (a maximum of 1.5-fold), when enzyme activities increase two to six times (depending on the enzyme). During the entire period of increase in glyoxysomal enzyme activities, no ultrastructural evidence was found for glyoxysome formation or destruction. Our data, in contrast to some proposals in the literature, indicate that cottonseed glyoxysomes form during seed maturation, then develop following seed imbibition into pleomorphic organelles by posttranslational accumulation of proteins from the cytosol and transfer of membrane components probably from the endoplasmic reticulum.

Synthesis and posttranslational segregation of glyoxysomal isocitrate lyase from castor bean endosperm

Total polyadenylated RNA isolated from 3-day-old germinating castor bean endosperm was translated in the rabbit reticulocyte lyase cell-free protein-synthesizing system in either the presence or absence of canine pancreatic microsomal vesicles. Isocitrate lyase was immunoprecipitated from the translational products using rabbit antibodies raised against the purified glyoxysomal enzyme. Isocitrate lyase synthesized in vitro had the same apparent molecular weight as the authentic glyoxysomal enzyme irrespective of whether synthesis occurred in the presence or absence of microsomes. Furthermore, in contrast to several proteins encoded by mRNA isolated from maturing seed endosperm tissue, isocitrate lyase was not cotranslationally segregated into the lumen of the microsomal vesicles. Intact 3-day-old endosperm tissue was labelled in vivo by adding [35S]methionine and at various times the tissue was homogenized and the cellular organelles fractionated. Separated fractions were assayed for newly synthesized isocitrate lyase. Immunoreactive product was initially detected in the cytosolic fraction and subsequently began to accumulate in glyoxysomes. The results indicate that glyoxysomal isocitrate lyase is synthesized on free polysomes and released into the cytosol. Transfer to the matrix of the glyoxysome involves a posttranslational membrane translocation step which is not accompanied by proteolytic cleavage of the polypeptide chain.