The intrinsic factor-vitamin B12 receptor and target of teratogenic antibodies is a megalin-binding peripheral membrane protein with homology to developmental proteins

The present report shows the molecular characterization of the rat 460-kDa epithelial glycoprotein that functions as the receptor facilitating uptake of intrinsic factor-vitamin B12 complexes in the intestine and kidney. The same receptor represents also the yolk sac target for teratogenic antibodies causing fetal malformations in rats. Determination of its primary structure by cDNA cloning identified a novel type of peripheral membrane receptor characterized by a cluster of eight epidermal growth factor type domains followed by a cluster of 27 CUB domains. In accordance with the absence of a hydrophobic segment, the receptor could be released from renal cortex membranes by nonenzymatic and nonsolubilizing procedures. The primary structure has no similarity to known endocytic receptors but displays homology to epidermal growth factor and CUB domain proteins involved in fetal development, e.g. the bone morphogenic proteins. Electron microscopic immunogold double labeling of rat yolk sac and renal proximal tubules demonstrated subcellular colocalization with the endocytic receptor megalin, which is expressed in the same epithelia as the 460-kDa receptor. Furthermore, megalin affinity chromatography and surface plasmon resonance analysis revealed a calcium-dependent high affinity binding of the 460-kDa receptor to megalin, which thereby may mediate its vesicular trafficking. Due to the high number of CUB domains, accounting for 88% of the protein mass, we propose the name cubilin for the novel receptor.

Spectrofluorimetric study of porphyrin incorporation into membrane models--evidence for pH effects

The effects of hydrophobicity and charges of dicarboxylic porphyrins upon their interactions with membrane model systems are investigated. Four protonation steps are evidenced from fluorescence emission studies of hematoporphyrin IX and its more hydrophobic parent compound lacking of alcoholic chain, deuteroporphyrin IX. They are attributed to the successive protonations of the inner nitrogens of the porphyrin cycle (pK = 4.7 and 2.9 for hematoporphyrin and 4.4 and 2.7 for deuteroporphyrin) and successive deprotonations of propionic groups (pK approximately equal to 5.0 and 5.5 for hematoporphyrin and 5.4 and 6.0 for deuteroporphyrin). These porphyrins, as well as their dimethyl ester forms, are shown to incorporate as monomers into the hydrophobic bilayer of egg phosphatidylcholine small unilamellar vesicles, although the esterified forms are highly aggregated in aqueous solutions. In the case of the non-esterified forms, the incorporation of the porphyrins into the lipidic bilayer is reversible and strongly pH-dependent. A theoretical model is presented which takes into account the various protonation steps and the partition equilibria of the porphyrin between the vesicle lipidic phase and the water medium. The neutral form of the porphyrin (i.e., carboxylic groups protonated) presents the higher affinity, with constants of K approximately equal to 2 X 10(5) and K approximately equal to 6 X 10(6) M-1 (relative to lipid concentration) for hematoporphyrin and deuteroporphyrin, respectively. Protonation of one inner nitrogen leading to the monocationic form is sufficient to prevent incorporation into the hydrophobic bilayer. On the other hand, deprotonation of the peripheral propionic chains leading to anionic forms is less effective. These interactions between vesicles and porphyrins lead to shifts of the apparent pK of nitrogens and carboxylic groups, the latter one being now in the range of physiological pH. These results are discussed with regards to the hypothesis of a possible role of pH in the preferential uptake of porphyrins by tumors.

Physical chemistry of porphyrins and their interactions with membranes: the importance of pH

The physicochemical properties of dicarboxylic porphyrins are examined with particular emphasis on acid-base equilibria. Interactions with membranes appear to be dominated by the charge of the propionic acid chains and the hydrophobic character of other peripheral substituents. A physicochemical basis for the effect of pH on the incorporation of porphyrins within membranes is given. The effect of lowered tumour pH on the incorporation of porphyrins within membranes and on porphyrin retention is discussed.

Singlet molecular oxygen quenching by saturated and unsaturated fatty-acids and by cholesterol

The rate constants of molecular singlet oxygen quenching by saturated and unsaturated fatty-acids and by cholesterol-membrane critical components - membrane critical components - have been measured by time resolved detection of the 1270 nm phosphorescence of singlet molecular oxygen [O2(1deltag)]. We have determined (i) an increment of 5.7 x 10(2)M(-1)s(-1) per -CH2- in C6D6 and CD3OD for saturated fatty acids between C4 and C20, (ii) an increment of 3 x 10(4)M(-1)s(-1) per non-conjugated cis-double bond for C18 unsaturated fatty acids, identical in C6D6 and DC3OD, (iii) a lower quenching rate constant by a factor of 2.7 for the trans-C16 and trans-C18 as compared to the corresponding cis-monounsaturated fatty acids, (iv) a rate constant of O2x(1deltag) quenching by cholesterol of 5.7 x 10(4)M(-1)s(-1) in benzene. These rate constants are compared to those obtained for other membrane cellular components.

Kinetic and equilibrium studies of porphyrin interactions with unilamellar lipidic vesicles

The interaction of deuteroporphyrin with dimyristoylphosphatidylcholine unilamellar vesicles of various sizes (ranging from 38 to 222 nm) has been studied using a stopped flow with fluorescence detection. Beside the kinetics of porphyrin incorporation into vesicles, the transfer of porphyrin from vesicles to human serum albumin has been investigated both experimentally and theoretically. The effects of both vesicle and albumin concentrations indicate that the transfer proceeds through the aqueous phase. It is governed by the rate of incorporation of porphyrin into the outer vesicle hemileaflet (kon), by the exit to the bulk aqueous medium (koff), and by the association (kas) and dissociation (kdis) constants relative to albumin. In both systems studied, a slower transbilayer flip-flop accounts for the biphasic character of the kinetics. This model is strongly supported by the effects of vesicle size, temperature, and cholesterol. The dependence of kon on the vesicle size indicates that the incorporation is diffusion controlled. The constant koff is found to be closely coupled to the phase state of the bilayer. The transbilayer flip-flop rate constant is approximately the same in both directions (approximately 0.4 s-1 at 32 degrees C and pH 7.4). It is strongly affected by the presence of cholesterol in vesicles and by the temperature, with a sharp enhancement around the phase transition. With the exception of very small vesicles obtained by sonication, no influence of the vesicle size on the flip-flop rate was observed. An accelerating effect of tetrahydrofuran, used to improve the solubility of porphyrin, has been noted. Steady-state measurements and kinetics results were in excellent agreement. The interest of systems involving albumin as a scavenger to extract important rate constants, is emphasized.

Prognosis of stage II colon cancer by non-neoplastic mucosa gene expression profiling

We have assessed the possibility to build a prognosis predictor (PP), based on non-neoplastic mucosa microarray gene expression measures, for stage II colon cancer patients. Non-neoplastic colonic mucosa mRNA samples from 24 patients (10 with a metachronous metastasis, 14 with no recurrence) were profiled using the Affymetrix HGU133A GeneChip. Patients were repeatedly and randomly divided into 1000 training sets (TSs) of size 16 and validation sets (VS) of size 8. For each TS/VS split, a 70-gene PP, identified on the TS by selecting the 70 most differentially expressed genes and applying diagonal linear discriminant analysis, was used to predict the prognoses of VS patients. Mean prognosis prediction performances of the 70-gene PP were 81.8% for accuracy, 73.0% for sensitivity and 87.1% for specificity. Informative genes suggested branching signal-transduction pathways with possible extensive networks between individual pathways. They also included genes coding for proteins involved in immune surveillance. In conclusion, our study suggests that one can build an accurate PP for stage II colon cancer patients, based on non-neoplastic mucosa microarray gene expression measures.

Rate constants for one-electron oxidation by the CF3O2., CCl3O2., and CBr3O2. radicals in aqueous solutions

The peroxyl radicals CF3O2., CCl3O2. and CBr3O2. were produced by radiolysis of aerated aqueous-alcohol solutions of CF3Br, CF3Cl, CCl4 or CBr4. Kinetic spectrophotometric pulse radiolysis experiments were carried out in the presence of various substrates: urate, ascorbate, xanthine, hydroquinone, p-methoxyphenol, phenol and chlorpromazine. Absolute rate constants for one-electron oxidation of these substrates by the alkylperoxyl radicals were found to vary from less than 10(5) to greater than 10(9) M-1 s-1, depending to some extent on the redox potential of the substrate. For all substrates the order of reactivity was CF3O2. greater than CBr3O2. greater than CCl3O2. . Because of its high reactivity, CF3O2., may have deleterious effects on biological systems. Its likely environmental precursor, CF3Br, which is used as a fire extinguisher and a refrigerant, was found to be reduced by a ferrous porphyrin model for cytochrome P-450 only very slowly and thus is not expected to have a major toxic effect if inhaled.

Comparison of the cDNA and amino acid sequences of lipoprotein lipase in eight species

By aligning nucleotide and amino acid sequences of lipoprotein lipase in eight species (man, pig, cow, sheep, mouse, rat, guinea-pig and chicken), we found that the main domains (catalytic, N-glycosylation and putative heparin binding sites) are well conserved. The longest identical amino acid chain was encoded by a sequence between the end of exon 2 and the beginning of exon 3, emphasizing the importance of this region which encodes the beta 5-loop of the active site, among other domains. Exon 10 is entirely untranslated in the seven mammals studied here and contains species-characteristic deletions, insertions or elements rich in A or A + T. In chicken, the beginning of exon 10 is translated. These eight previously unreported alignments could be a useful tool for further studies on LPL function.

Kinetics of the interactions of a dicarboxylic porphyrin with unilamellar lipidic vesicles: interplay between bilayer thickness and pH in rate control

The transfer of a dicarboxylic porphyrin from phosphatidylcholine fluid-phase unilamellar vesicles towards albumin is studied focusing on bilayer thickness and pH effects. The kinetics of this process yield the rate constants for the porphyrin flip-flop from the inner to the outer hemileaflet and its exit towards aqueous medium. Phospholipids with monounsaturated 14-22 carbon chains are used. Interplay between bilayer thickness and pH for the control of the rate constants is observed. This results in the amplification, at physiological pH, of the effect of membrane thickness on the flip-flop and exit rates as compared to pH 8.5 and 6.5. These data are explained by the degree of porphyrin burying within the bilayer resulting from a compromise between favorable hydrophobic interactions with the hydrocarbon phase and unfavorable penetration of the polar carboxylic chains. The balance between the two effects depends particularly on the neutralization of one carboxylic chain. Considering the bilayer hydrophobicity profile and the porphyrin size, the optimization of hydrophobic interactions appears dependent on the bilayer thickness. The flip-flop and the exit are governed by neutralization and deprotonation of the carboxylic chains, respectively, the rate of these proton exchanges being dependent on the porphyrin location within the bilayer.

Nuclear magnetic resonance and biochemical measurements of glucose utilization in the cone-dominant ground squirrel retina

PURPOSE

To provide quantitative information on glucose utilization in cone-dominant ground squirrel retinas.

METHODS

Ground squirrel eyecups were incubated in medium containing (14)C-glucose, and the production of (14)CO(2) was measured. Measurements were also made of lactic acid production (glycolysis). Nuclear magnetic resonance (NMR) was used to track metabolites generated from (13)C-1 glucose.

RESULTS

Ground squirrel eyecups produced lactate at a high rate and exhibited normal histology. Light-adaptation reduced glycolysis by 20%. Ouabain decreased glycolysis by 25% and decreased (14)CO(2) production by 60%. Blockade of glutamate receptors had little effect on the glycolysis and (14)CO(2) produced. When metabolic responses were restricted to photoreceptors, light caused a 33% decrease in (14)CO(2) production. The rate of (14)CO(2) production was less than 10% of lactate production. Lactate was the major product formed from (13)C-glucose. Other (13)C-labeled compounds included glutamate, aspartate, glutamine, alanine, taurine, and GABA. Lactate was the only product detected in the medium bathing the ground squirrel retinas. The rod-dominant rat retina exhibited a similar pattern of metabolites formed from glucose.

CONCLUSIONS

Lactate, not CO(2), is the major product of glucose metabolism in both ground squirrel and rat retinas. Active Na(+) transport, however, depends more on ATP produced by mitochondria than by glycolysis. A relatively high fraction of ATP production from glycolysis and glucose oxidation continues in the absence of active Na(+) pumping and glutamatergic transmission. Major neurotransmitters are synthesized from the aerobic metabolism of glucose; anoxia-induced impairment in retinal synaptic transmission may be due to depletion of neurotransmitters.

Interactions of dicarboxylic porphyrins with unilamellar lipidic vesicles: drastic effects of pH and cholesterol on kinetics

The effect of pH on the transfer of deuteroporphyrin from dimyristoylphosphatidylcholine (DMPC) unilamellar vesicles to human serum albumin is investigated using a stopped-flow with fluorescence detection. The kinetics of this process allows for the determination of the rate constants for the porphyrin exist from the outer vesicle layer to the bulk aqueous medium (koff), and for its movement from the inner to the outer vesicle layer (kto). Both koff and kto are found to strongly depend on the pH. The observed behavior can be described by classical titration curves and is most likely due to protonation equilibria involving the two carboxylic groups of the porphyrin. A pH increase accelerates the exist of the porphyrin. The reverse effect is observed for its movement through the bilayer. The presence of cholesterol in the DMPC bilayer also strongly affects the interactions of the porphyrin with the vesicles. The rate constant kto is dramatically reduced by increasing the cholesterol content. An irregularity is noted around 10-20 mol % cholesterol. The results are discussed in relation to the preferential uptake of porphyrins by tumors, a basis of photodynamic therapy, and to possible pH-mediated relocalization of porphyrins among subcellular structures.

The lipid peroxidation model for halogenated hydrocarbon toxicity. Kinetics of peroxyl radical processes involving fatty acids and Fe(III) porphyrins

The toxicity of halogenated alkanes originates from their metabolism by cytochrome P-450 which leads to the formation of reactive intermediates. In particular, peroxyl radicals derived from the halogenated compounds are believed to induce peroxidative chain degradation of lipids. To examine this hypothesis, radical reactions in a system involving FeIII-deuteroporphyrin as a model of cytochrome P-450, fatty acids or cholesterol, and carbon tetrachloride or the anesthetic agent halothane are studied by means of pulse radiolysis. It is shown that haloperoxyl radicals react with the fatty acids in competition with their reaction with the ferriporphyrin. Moreover, the secondary fatty acid peroxyl radicals also react efficiently with the porphyrin. A model for halogenated alkane toxicity is discussed in terms of these new findings. The importance of local oxygen concentration and structural arrangement of fatty acids around cytochrome P-450 are emphasized.

Sequence of rat lipoprotein lipase-encoding cDNA

A rat lipoprotein lipase (LPL)-encoding cDNA (LPL) has been entirely sequenced and compared to the sequences of all the LPL cDNAs reported in other species. As expected, high homology was found between the coding exons. The putative catalytic triad, Ser132, Asp156, His241, according to human numbering, is conserved in rat. As is the case in mouse, an Asn444 present in human LPL is also missing. The major divergences between human, mouse and rat LPLs were observed in the untranslated exon 10, where (i) the rat cDNA exhibits a 157-bp insertion and an 81-bp deletion relative to human; (ii) neither the B1 repeat nor the homopurine stretch reported in mouse can be recognized, and (iii) the rat cDNA displays several A+T-rich stretches.

Kinetic and equilibrium studies of incorporation of di-sulfonated aluminum phthalocyanine into unilamellar vesicles

The interactions of cis-di-sulfonated aluminum phthalocyanine (PcS(2)Al) with dimyristoylphosphatidylcholine (DMPC) unilamellar vesicles have been investigated by fluorescence spectroscopy. At pH 7.0, PcS(2)Al incorporates into the vesicles with a high affinity constant (2.7x10(6) M(-1), in terms of phospholipid concentration). The fluorescence changes following rapid mixing of PcS(2)Al with vesicles are biphasic. The first phase is attributed to the entry of PcS(2)Al into the vesicles, as deduced from the linear dependence of the rate upon lipid concentration. More surprisingly, this rate is strongly pH dependent with a marked maximum around pH 7.3, a result interpreted in terms of the coordination state of the aluminum ion in aqueous solutions. At this pH, a hydroxide ion neutralizes the residual positive charge of the metal ion that remains unbalanced after coordination by the phthalocyanine cycle. A water molecule is likely to complete the metal coordination sphere. Only this form, PcAl(+)(OH(-))(OH(2)), with an uncharged core is quickly incorporated into the vesicles. The protonation of OH(-) or the deprotonation of the coordinated H(2)O leading to a positively or negatively charged core, respectively, account for the observed pH effect. Studies on the effect of cholesterol addition and exchange of PcS(2)Al between vesicles and albumin all indicate the absence of transfer of the phthalocyanine between the vesicle hemileaflets, a result expected from the presence of the two negatively charged sulfonated groups at the ring periphery. Instead, the slower kinetic phase is likely due to the movement of the phthalocyanine becoming more buried within the outer leaflet upon the loss of the water molecule coordinated to the aluminum ion.

Fundamental aspects in tumor photochemotherapy: interactions of porphyrins with membrane model systems and cells

Some molecular aspects underlying photochemotherapy and photodiagnosis of tumors with porphyrins are reviewed. The nature of the clinically used photosensitizer HpD is first presented along with structures of molecules found to be efficient in vitro. The possible role of pH in the preferential retention of dicarboxylic porphyrins by tumors is discussed in light of results obtained with membrane models. The uptake of dicarboxylic porphyrins by cells most likely involves passive mechanisms. Cell photoinactivation using a purified porphyrin does not depend upon the incubation time but only on the intracellular concentration of the dye. This likely reflects a poor specificity of the photoinactivation processes with regard to the cellular localization of the dye. The properties which should be presented by more efficient photosensitizers are discussed.