Serum homocysteine and risk of coronary heart disease and cerebrovascular disease in elderly men: a 10-year follow-up

Hyperhomocysteinemia is an independent risk factor for atherosclerotic disease in the middle-aged. We investigated whether a high serum homocysteine level is a risk factor for vascular disease in 878 elderly men (mean age at baseline, 71.5 years; range, 64 to 84 years) in a population-based, representative cohort followed up for 10 years in Zutphen, the Netherlands. Thirty-one percent had nonfasting homocysteine levels >/=17 micromol/L. After adjustment for other major risk factors, high homocysteine levels at baseline (the third compared with the first tertile) were associated with an increased baseline prevalence of myocardial infarction (odds ratio [OR], 1.81; 95% confidence interval [CI], 1.07 to 3.08; P for trend, 0.03) and with a marginally significant increase in the risk of dying of coronary heart disease (relative risk [RR], 1.58; 95% CI, 0.93 to 2.69; P for trend, 0.09) but not with an increased risk of first-ever myocardial infarction. In addition, high homocysteine levels at baseline were associated with an increased baseline prevalence of stroke (OR, 4.61; 95% CI, 1.79 to 11.89; P for trend, 0.002) and with an increased risk of dying of cerebrovascular disease in subjects without hypertension (RR, 6.18; 95% CI, 2.28 to 16.76) but not in those with hypertension. High homocysteine levels were associated with an increased risk of first-ever stroke among normotensive subjects that was not statistically significant (RR, 1. 77 [95% CI, 0.83 to 3.75; P for trend, 0.14]). In a general population of elderly men, a high homocysteine level is common and is strongly associated with the prevalence of coronary heart disease and cerebrovascular disease. It is a strong predictive factor for fatal cerebrovascular disease in men without hypertension but less so for coronary heart disease.

Hyperhomocysteinemia and atherothrombotic disease

Hyperhomocysteinemia is an independent risk factor for atherothrombotic disease. The mechanism by which homocysteine induces atherosclerosis and thrombosis is not fully understood. Data on arterial histology in humans with homocystinuria and mild hyperhomocysteinemia are limited. In vitro studies as well as studies in animals and humans indicate that hyperhomocysteinemia induces dysfunction of the vascular endothelium, with loss of endothelium-dependent vasodilation and endothelial antithrombotic properties, and proliferation of vascular smooth muscle cells, which are key processes in current models of atherogenesis and thrombosis. One of the hypotheses is that homocysteine can lead to cellular dysfunction through a mechanism involving oxidative damage but future studies in humans are needed to confirm this. Studies in hyperhomocysteinemic vascular patients have shown that endothelial antithrombotic properties appear to be more severely impaired than in similar patients with normohomocysteinemia. Furthermore, impaired endothelium-dependent vasodilation has been observed in clinically healthy hyperhomocysteinemic subjects in whom no abnormalities were found in endothelial antithrombotic properties. Future studies involving homocysteine-lowering treatment in hyperhomocysteinemic patients with vascular disease and in clinically healthy hyperhomocysteinemic subjects are necessary to investigate the mechanisms by which homocysteine causes atherothrombotic disorders in humans.

Port-A-Cath usage in children with haemophilia: experience of 53 cases

Experience of the Port-A-Cath implantable venous access system in 53 children with severe or moderate haemophilia A or B from seven centres in five countries is reviewed. The cumulative duration of follow-up was 1578 months (median 30 months, range 1-114). Of the devices implanted, 70% (37/53) were used without complications (median follow-up 32 months; range 1-114) and the remaining 30% (16/53) were associated with various types of complication: infection, bacteraemia or septicaemia in 56% (9/16) of cases, i.e. a rate of 0.07 per follow-up year or 0.19 per 1000 patient days, or various technical complications occurring after a median of 32 months (range 4-75) of uncomplicated use in the remaining 44% (7/16). Of the patients with inhibitors, 64% (7/11) manifested complications. Both doctors and parents considered that the Port-A-Cath device can be used with an acceptable frequency and severity of complications, and that it enables regular prophylactic or on-demand home treatment of children with haemophilia to be begun at an early age.

Toxicokinetics of an environmentally relevant mixture of dioxin-like PHAHs with or without a non-dioxin-like PCB in a semi-chronic exposure study in female Sprague Dawley rats

Female Sprague Dawley rats were treated subcutaneously for 20 weeks with an environmentally relevant mixture of dioxin-like PHAHs with (PHAH+) or without (PHAH-) 2,2',4,4',5,5'-hexachlorobiphenyl. The hepatic retention (% of given dose) of the various PHAH congeners differed considerably and in the following order: 2,3,4,7,8-pentachlorodibenzofuran (30.5-43.1%), 3,3',4,4',5-pentachlorobiphenyl (12.8-17.6%), 1,2,3,7,8-pentachlorodibenzo-p-dioxin (6.9-10.8%), 2,3,7,8-tetrachlorodibenzo-p-dioxin (3.2-4.5%), 2,3,3',4,4',5-hexachlorobiphenyl (1.0-1.7%), 2,2',4,4',5,5'-hexachlorobiphenyl (0.5-0.8%) and 2,3',4,4',5-pentachlorobiphenyl (0.2-0.4%). A decrease of the hepatic retention of 2,3,7,8-TCDD, 1,2,3,7,8-PeCDD and 2,3,4,7,8-PeCDF was found at increasing doses of the PHAH+ mixture. 2,2',4,4',5,5'-Hexachlorobiphenyl increased the hepatic retention (1.3-2 times) of all congeners in the PHAH+ group, compared to the TEQ equivalent dosed PHAH- group. No interactions were observed on the ethoxyresorufin-O-deethylase activity.

Laparoscopic Nissen fundoplication for the treatment of gastroesophageal reflux disease (GERD). Surgery after extensive conservative treatment

BACKGROUND

A prospective study was conducted to evaluate the physiologic and clinical consequences of laparoscopic Nissen fundoplication (LNF), using strict indications for surgery.

METHODS

From 1992 to 1997, 50 patients underwent LNF. Indications for operative treatment were either failure of conservative treatment or foresight to see long-term use of strong acid suppressive therapy. Patients were evaluated by barium esophagogastric study (BES), esophagoscopy, 24-h pH monitoring (pHM), stationary esophageal manometry, gastric-emptying studies (GES), pancreatic polypeptide stimulation test (PPT) and clinical evaluation using questionnaires.

RESULTS

Perioperative complications necessitated conversion to laparatomy in two cases, and there was no mortality. Severe dysphagia resulted in reoperation in two patients. The average maximum lower esophageal sphincter pressure (MLESP) increased from 6. 1 mmHg to 12.7 mmHg. Endoscopy showed improved grading of the esophagitis, and the total percentage of pH less than 4 during 24 h decreased from a mean of 9.2 to 0.95. Three patients demonstrated impaired PPTs postoperatively; two had (mild) diarrhea. The overall success rate after the operation was 90%.

CONCLUSIONS

The results of LNF in a limited number of patients with severe and/or resistant gastroesophageal reflux disease (GERD) receiving continuous medical treatment with proton pump inhibitors (PPIs) on a maintenance base are comparable with LNF results in centers with a more liberal policy concerning indications for LNF surgery.

Transgastric endoluminal laparoscopic resection of a duodenal polyp

Transgastric endoluminal surgery is a laparoscopic procedure used to treat gastric lesions such as polyps and carcinoma in situ. This approach can also be used to treat proximal duodenal problems such as bleeding and polyps. Our case was a young patient with bleeding from a benign duodenal polyp, localized 3 cm from the papilla of Vater. Two trocars were introduced by laparoscopy into the stomach. Guided by the flexible gastroscope, the polyp (a 6-cm Brunner's benign hamartoma) was resected by means of the endostaplers. Transgastric endoluminal surgery can be used to extirpate benign lesions in the proximal duodenum. The use of a flexible gastroscope can be very useful because of its adaptability to the anatomy.

The cytosolic DnaJ-like protein djp1p is involved specifically in peroxisomal protein import

The Saccharomyces cerevisiae DJP1 gene encodes a cytosolic protein homologous to Escherichia coli DnaJ. DnaJ homologues act in conjunction with molecular chaperones of the Hsp70 protein family in a variety of cellular processes. Cells with a DJP1 gene deletion are viable and exhibit a novel phenotype among cytosolic J-protein mutants in that they have a specific impairment of only one organelle, the peroxisome. The phenotype was also unique among peroxisome assembly mutants: peroxisomal matrix proteins were mislocalized to the cytoplasm to a varying extent, and peroxisomal structures failed to grow to full size and exhibited a broad range of buoyant densities. Import of marker proteins for the endoplasmic reticulum, nucleus, and mitochondria was normal. Furthermore, the metabolic adaptation to a change in carbon source, a complex multistep process, was unaffected in a DJP1 gene deletion mutant. We conclude that Djp1p is specifically required for peroxisomal protein import.

RNA editing in the free-living bodonid Bodo saltans

In parasitic kinetoplastid protozoa, mitochondrial (mt) mRNAs are post-transcriptionally edited by insertion and deletion of uridylate residues, the information being provided by guide (g) RNAs. In order to further explore the role and evolutionary history of this process, we searched for editing in mt RNAs of the free-living bodonid Bodo saltans. We found extensive editing in the transcript for NADH dehydrogenase (ND) subunit 5, which is unedited in trypanosomatids. In contrast, B.saltans cytochrome c oxidase (cox) subunit 2 and maxicircle unidentified reading frame (MURF) 2 RNAs display limited editing in the same regions as their trypanosomatid counterparts. A putative intramolecular cox2 gRNA and the gene for gMURF2-I directing the insertion of only one U in the 5' editing domain of MURF2 RNA, are conserved in B.saltans. This lends (further) evolutionary support to the proposed role of these sequences as gRNAs. Phylogenetic analysis showed that B.saltans is more closely related to trypanosomatids than the cryptobiids Trypanoplasma borreli and Cryptobia helicis, in line with the trypanosomatid-like cox2 and MURF2 RNA editing patterns. Nevertheless, other features like the apparent absence of a catenated mtDNA network, are shared with bodonid and cryptobiid species. ND5 RNA editing may represent yet another example of editing 'on the way out' during kinetoplastid evolution, but in view of the fact that cox2 RNA is unedited in T. borreli and C.helicis, we infer that the editing of this RNA may have arisen relatively recently. Our results provide the first examples of RNA editing in a free-living kinetoplastid, indicating that there is no direct link between U-insertion/deletion editing and a parasitic lifestyle.

Peroxisomal beta-oxidation of polyunsaturated fatty acids in Saccharomyces cerevisiae: isocitrate dehydrogenase provides NADPH for reduction of double bonds at even positions

The beta-oxidation of saturated fatty acids in Saccharomyces cerevisiae is confined exclusively to the peroxisomal compartment of the cell. Processing of mono- and polyunsaturated fatty acids with the double bond at an even position requires, in addition to the basic beta-oxidation machinery, the contribution of the NADPH-dependent enzyme 2,4-dienoyl-CoA reductase. Here we show by biochemical cell fractionation studies that this enzyme is a typical constituent of peroxisomes. As a consequence, the beta-oxidation of mono- and polyunsaturated fatty acids with double bonds at even positions requires stoichiometric amounts of intraperoxisomal NADPH. We suggest that NADP-dependent isocitrate dehydrogenase isoenzymes function in an NADP redox shuttle across the peroxisomal membrane to keep intraperoxisomal NADP reduced. This is based on the finding of a third NADP-dependent isocitrate dehydrogenase isoenzyme, Idp3p, next to the already known mitochondrial and cytosolic isoenzymes, which turned out to be present in the peroxisomal matrix. Our proposal is strongly supported by the observation that peroxisomal Idp3p is essential for growth on the unsaturated fatty acids arachidonic, linoleic and petroselinic acid, which require 2, 4-dienoyl-CoA reductase activity. On the other hand, growth on oleate which does not require 2,4-dienoyl-CoA reductase, and NADPH is completely normal in Deltaidp3 cells.

Identification of residues in the Gla-domain of human factor IX involved in the binding to conformation specific antibodies

The binding of Ca2+ induces a conformational change in factor IX which can be monitored with conformation specific antibodies. Anti-FIX:Mg(II) antibodies recognize a conformational epitope (FIX') that can be induced by several metal ions such as Ca2+, Mg2+, Mn2+ and Ba2+, while anti-FIX:Ca(II) antibodies recognize a conformational epitope (FIX*) that can be only induced by Ca2+ and Sr2+ ions (Liebman et al., J. Biol. Chem., vol. 262 (1987) pp. 7605-7612). The latter conformation is essential for the function of factor IX. In this study we tried to identify residues in the Gla-domain of factor IX which are involved in binding to anti-factor IX:Mg(II) and anti-factor IX:Ca(II) antibodies. For this we substituted residues in recombinant human factor IX for those of factor X or factor VII. The substitution of residues 1-40 of factor IX by those of factor VII eliminated binding to both types of antibodies. Re-introduction of factor IX specific residues increased the binding to conformation specific anti-factor IX antibodies, but reduced the binding to conformation specific anti-factor VII antibodies, indicating that the structural integrity of the Gla-domain was not seriously affected by the mutations. We provide evidence that residues 33, 39 and 40 of human factor IX are important for binding to anti-factor IX:Mg(II) antibodies, while residues 1-11 are important for binding to anti-factor IX:Ca(II) antibodies.

Induction of chromosomal aberrations (unstable and stable) by inhibitors of topoisomerase II, m-AMSA and VP16, using conventional Giemsa staining and chromosome painting techniques

Frequencies of symmetrical and asymmetrical exchange aberrations induced by two inhibitors of topoisomerase II, namely, 4'-(9-acridinylamino) methanesulfon-m-anisidide (m-AMSA) and etoposide (VP16), were estimated in human peripheral blood lymphocytes. The aberrations were scored using conventional Giemsa staining and fluorescence in situ hybridization (FISH) techniques, using chromosome-specific DNA libraries. Stable aberrations (translocations) were detected using two cocktails of DNA libraries specific for three chromosomes, namely 1, 3 and X and 2, 4 and 8, representing approximately 40% of the whole human genome. The frequencies of dicentrics and translocations increased in a dose-dependent manner, however, m-AMSA was found to be a more potent inducer of chromosomal aberrations in comparison with VP16 (at concentrations at which comparable frequencies of aberrations were induced) by 20- to 30-fold. When corrected for DNA content of chromosomes in each cocktail, a higher frequency of translocations with the cocktail consisting of chromosomes 2, 4 and 8 in comparison with 1, 3 and X was evident. The genomic translocation frequency calculated from chromosome painting analysis for m-AMSA exceeded that estimated for dicentrics by approximately 2-fold. However, for VP16 almost equal frequencies of both types of chromosome exchange were found.

Dominant C2 domain epitope specificity of inhibitor antibodies elicited by a heat pasteurized product, factor VIII CPS-P, in previously treated hemophilia A patients without inhibitors

From June, 1990, to November, 1991, in The Netherlands and Belgium, 16 previously treated severe hemophilia A patients (PTP) developed inhibitors after exposure to factor VIII CPS-P, a new heat pasteurized product. A previously untreated patient (PUP) also developed an inhibitor to CPS-P. In inhibitor neutralization assays with recombinant fVIII C2 and A2 domain polypeptides, plasmas from 14 PTPs were > or = 79% neutralized by C2 and < 10% by A2, but the PUP plasma was partially neutralized by C2 (48%) and A2 (28%). Immunoprecipitation assays of the PTP and PUP plasmas with the fVIII heavy chain and with recombinant C2 and A3-C1 polypeptides confirmed that the C2 dominant immune response to CPS-P was found only in the PTPs. Competition of the binding of 2 inhibitors to 125I-CPS-P by unlabeled CPS-P and another plasma fVIII was similar, demonstrating that the antibody response was not directed to epitopes only present in CPS-P. We propose that the immunogenicity of the CPS-P C2 domain was altered by heat pasteurization.

Overexpression of Pex15p, a phosphorylated peroxisomal integral membrane protein required for peroxisome assembly in S.cerevisiae, causes proliferation of the endoplasmic reticulum membrane

We have cloned PEX15 which is required for peroxisome biogenesis in Saccharomyces cerevisiae. pex15Delta cells are characterized by the cytosolic accumulation of peroxisomal matrix proteins containing a PTS1 or PTS2 import signal, whereas peroxisomal membrane proteins are present in peroxisomal remnants. PEX15 encodes a phosphorylated, integral peroxisomal membrane protein (Pex15p). Using multiple in vivo methods to determine the topology, Pex15p was found to be a tail-anchored type II (Ncyt-Clumen) peroxisomal membrane protein with a single transmembrane domain near its carboxy-terminus. Overexpression of Pex15p resulted in impaired peroxisome assembly, and caused profound proliferation of the endoplasmic reticulum (ER) membrane. The lumenal carboxy-terminal tail of Pex15p protrudes into the lumen of these ER membranes, as demonstrated by its O-glycosylation. Accumulation in the ER was also observed at an endogenous expression level when Pex15p was fused to the N-terminus of mature invertase. This resulted in core N-glycosylation of the hybrid protein. The lumenal C-terminal tail of Pex15p is essential for targeting to the peroxisomal membrane. Furthermore, the peroxisomal membrane targeting signal of Pex15p overlaps with an ER targeting signal on this protein. These results indicate that Pex15p may be targeted to peroxisomes via the ER, or to both organelles.

Ethoxyresorufin-O-deethylase (EROD) inducing potencies of planar chlorinated aromatic hydrocarbons in primary cultures of hepatocytes from different developmental stages of the chicken

In vitro induction of ethoxyresorufin O-deethylase (EROD) activity in cell cultures is an extensively validated tool for measuring overall potencies of mixtures of halogenated aromatic hydrocarbons (HAHs) in samples from the abiotic or biotic environment. For risk assessment with special attention to effects in wild birds, an assay was developed that makes use of chicken embryo hepatocytes. However, it was questioned whether compound-specific responses are consistent at the various developmental stages. The results of our present study show that there are considerable differences between early and late embryonal and post-hatching stages. The induction of EROD was measured in primary chicken hepatocyte cultures. The cells were isolated at day 14 and day 19 of embryonal development and at day 1 post hatching. Hepatocytes were exposed in vitro to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 3,3',4,4',5-pentachlorobiphenyl (PCB 126, IUPAC nomenclature) and 2,3',4,4',5-pentachlorobiphenyl (PCB 118). The respective compounds were chosen as representatives for dioxins, furans, non-ortho PCBs, and mono-ortho PCBs. These groups of chemicals have been identified as environmental contaminants with major dioxin-like effects that are mediated by a common receptor, the arylhydrocarbon (Ah) receptor. At all developmental stages, TCDF was more potent than TCDD. Relative potencies (RP = EC50TCDD/EC50HAH) decreased in the order TCDF < TCDD < PCB 126 < PCB 118. Depending on the developmental stage, TCDF was 1.2 to 3.4 times more potent than TCDD. PCB 126 was equipotent or less potent by a factor of 3 than TCDD. PCB 118 was 100 to 300 times less potent than TCDD. Both the mean effective concentration (EC50) and the maximum EROD activity (Ymax) of all compounds were lower in hepatocyte cultures from 14-day-old embryos than those from 19-day-old embryos or 1-day-old hatchlings. RPs were comparable in 19-day-old embryos and in hatchlings, but significantly different in 14-day-old embryos.

Endothelial marker proteins in hyperhomocysteinemia

Hyperhomocysteinemia is associated with severe, premature atherosclerosis and thromboembolism. The mechanisms involved in the atherogenic and thrombotic complications of hyperhomocysteinemia are not understood. It has been suggested that hyperhomocysteinemia predisposes to atherosclerosis by injuring the vascular endothelium. Whether hyperhomocysteinemia is independently associated with changed endothelial function, either in the absence or the presence of clinically manifest atherosclerotic disease, is, however, not known. Therefore we investigated, both in patients with peripheral arterial occlusive disease and in healthy individuals, whether plasma protein markers of endothelial function differed between subjects with, and subjects without hyperhomocysteinemia. We studied 80 individuals under the age of 56 years: healthy individuals with (n = 20) and without (n = 20) hyperhomocysteinemia and patients with peripheral arterial occlusive disease with (n = 20) and without (n = 20) hyperhomocysteinemia. The following endothelium-derived proteins were measured as markers of endothelial cell function: von Willebrand factor (vWf) and von Willebrand factor propeptide (vWf: AgII), tissue-type plasminogen activator (tPA), plasminogen activator inhibitor-1 (PAI-1), cellular fibronectin (cFN) and thrombomodulin (TM). In addition we assessed C-reactive protein (CRP). vWf, vWf: AgII, tPA and CRP were significantly higher in the patients with peripheral arterial occlusive disease than in the healthy individuals. No differences in marker protein plasma levels were found between individuals with, and those without hyperhomocysteinemia, apart from vWf, which was significantly raised in hyperhomocysteinemic as compared to normohomocysteinemic patients. We did not find any evidence for an independent association between hyperhomocysteinemia and protein markers of endothelial cell function in healthy subjects.

High prevalence of hyperhomocysteinemia and asymptomatic vascular disease in siblings of young patients with vascular disease and hyperhomocysteinemia

Hyperhomocysteinemia (HHC) is associated with an increased risk of atherosclerotic vascular disease and may be inherited. Fasting and postmethionine HHC are independent risk factors that overlap to a limited extent. To study the familial occurrence of HHC, we investigated the prevalence of HHC (both fasting and after methionine) among 450 siblings of 167 consecutive young patients with vascular disease and postmethionine HHC. Furthermore, all subjects with postmethionine HHC (n = 125) were invited for noninvasive vascular testing; 101 (80.8%) agreed. Of those with a normal postmethionine plasma level (n = 325), we randomly selected 73 subjects for further studies; 53 agreed (72.6%). Thus, a total of 154 siblings underwent ultrasonography of the carotid arteries, measurement of ankle-brachial pressure indices at rest and after a treadmill exercise test, and exercise electrocardiographic stress testing. We observed HHC after methionine, fasting, or both, in 27.8% (95% CI, 23.7 to 31.9), 11.1% (CI, 8.2 to 14.0) and 8.7% (CI, 6.1 to 11.3) of the siblings. Abnormal peripheral, coronary, or carotid artery tests were observed in 35.7% (CI, 28.1 to 43.3), 7.1% (CI, 3.0 to 11.2), and 7.1% (CI, 3.0 to 11.2). Univariate and multivariate analyses revealed weak evidence of a relationship with homocysteine levels. In conclusion, we found a high prevalence of HHC and asymptomatic vascular disease in siblings of young patients with vascular (mainly peripheral arterial) disease and HHC. Our data raise the possibility that homocysteine does not play a major role in the early, asymptomatic phases of vascular disease, at least among siblings of young patients with vascular disease.

Synthesis, sorting, and processing into distinct isoforms of human macrophage chitotriosidase

Chitotriosidase, the human analogue of chitinases from non-vertebrate species, has recently been identified. The macrophage-derived enzyme is remarkably heterogeneous in molecular mass and isoelectric point. The synthesis and modification of the enzyme in cultured macrophages is reported. Chitotriosidase is synthesized as a 50-kDa protein with a pI of about 6.5 and 7.2. It is predominantly secreted, but in part processed into a 39-kDa form with a pI of 8.0 that accumulates in lysosomes. In the C-terminal extension of the 50-kDa chitotriosidase, sialic-acid containing O-linked glycans are present, causing its heterogeneous acidic isoelectric point. Chitotriosidase lacks N-linked glycans and the mechanism of routing to lysosomes proves to be distinct from that of soluble, N-glycosylated, lysosomal enzymes. It was observed that, in macrophages, alternative splicing generates a distinct chitotriosidase mRNA species, encoding a 40-kDa chitotriosidase that is C-terminally truncated. This enzyme is almost identical to the 39-kDa chitotriosidase formed from the 50-kDa precursor by proteolytic processing. It is concluded that the C-terminus present in the 50-kDa chitotriosidase, but absent in the 39-kDa isoform, was found to mediate tight binding to chitin. In the blood stream the secretory 50-kDa chitotriosidase occurs predominantly, whilst in tissues the 39-kDa form is also abundant. These findings are consistent with the data on the synthesis and processing of chitotriosidase in the cultured macrophage model.

Analysis of the carboxyl-terminal peroxisomal targeting signal 1 in a homologous context in Saccharomyces cerevisiae

Most peroxisomal matrix proteins contain a carboxyl-terminal tripeptide that directs them to peroxisomes. Within limits, these amino acids may be varied, without loss of function. The specificity of this peroxisomal targeting signal (PTS1) is remarkable considering its small size and its relaxed consensus sequence. Moreover, several peroxisomal proteins have a PTS1-like signal that does not fit the reported consensus sequence. Because many of these PTS1 variants seem to be functional in a species-dependent or protein context-dependent manner, we investigated the PTS1 requirements in a homologous context, using Saccharomyces cerevisiae and endogenous peroxisomal malate dehydrogenase (MDH3). Peroxisomal import of the MDH3-PTS1 variants was tested qualitatively by the ability to complement the Deltamdh3 mutant and quantitatively by subcellular fractionation. We observed efficient import of MDH3 into peroxisomes with a large variety of PTS1 tripeptides. Many of these variants do not fit the observed PTS1 requirements for heterologously expressed proteins, which suggests that additional domains in the protein may be of decisive importance whether or not a certain PTS1 variant is recognized by the components of the peroxisomal import machinery. Because we show that dimerization of MDH3 precedes import into the organelle, these domains are most likely conformational domains.

The SH3 domain of the Saccharomyces cerevisiae peroxisomal membrane protein Pex13p functions as a docking site for Pex5p, a mobile receptor for the import PTS1-containing proteins

We identified a Saccharomyces cerevisiae peroxisomal membrane protein, Pex13p, that is essential for protein import. A point mutation in the COOH-terminal Src homology 3 (SH3) domain of Pex13p inactivated the protein but did not affect its membrane targeting. A two-hybrid screen with the SH3 domain of Pex13p identified Pex5p, a receptor for proteins with a type I peroxisomal targeting signal (PTS1), as its ligand. Pex13p SH3 interacted specifically with Pex5p in vitro. We determined, furthermore, that Pex5p was mainly present in the cytosol and only a small fraction was associated with peroxisomes. We therefore propose that Pex13p is a component of the peroxisomal protein import machinery onto which the mobile Pex5p receptor docks for the delivery of the selected PTS1 protein.

Homocystinuria: what about mild hyperhomocysteinaemia?

Hyperhomocysteinaemia is associated with an increased risk of atherosclerotic vascular disease and thromboembolism, in both men and women. A variety of conditions can lead to elevated homocysteine levels, but the relation between high levels and vascular disease is present regardless of the underlying cause. Pooled data from a large number of studies demonstrate that mild hyperhomocysteinaemia after a standard methionine load is present in 21% of young patients with coronary artery disease, in 24% of patients with cerebrovascular disease, and in 32% of patients with peripheral vascular disease. From such data an odds ratio of 13.0 (95% confidence interval 5.9 to 28.1), as an estimate of the relative risk of vascular disease at a young age, can be calculated in subjects with an abnormal response to methionine loading. Furthermore, mild hyperhomo-cysteinaemia can lead to a two- or three-fold increase in the risk of recurrent venous thrombosis. Elevated homocysteine levels can be reduced to normal in virtually all cases by simple and safe treatment with vitamin B6, folic acid, and betaine, each of which is involved in methionine metabolism. A clinically beneficial effect of such an intervention, currently under investigation, would make large-scale screening for this risk factor mandatory.