Effect of iron status on the intestinal absorption of aluminum: a reappraisal

Kinetin--a multiactive molecule

Cytokinins are important adenine derivatives that serve as hormones to control many processes in plants. They were discovered as factors that promote cell division in tobacco tissue cultures and have been shown also to regulate several other developmental events. Kinetin which was isolated 50 years ago for the first time as a plant hormone, as well as other cytokinins isopentenyladenine, zeatin and benzylaminopurine induce callus (clusters of dedifferentiated plant cells) to redifferentiate into adventitious buds. Because of some similarities in the biological phenotypes of cancer and callus cells, cytokinins and especially kinetin, affect the differentiation of human cells through a common signal transduction system. Therefore, cytokinins found their way to use in molecular medicine.

Effect of the biologically active fragment of zonula occludens toxin, delta G, on the intestinal paracellular transport and oral absorption of mannitol

OBJECTIVE

Many therapeutically active agents experience low bioavailability upon oral administration due to low permeability, low solubility, interaction with efflux transporters or first pass metabolism. In general, absorption enhancers are agents that can modulate the paracellular permeability of drugs, thus, potentially increasing oral bioavailability. The objective of this study was to examine the effect of the active fragment of Zonula occludens toxin (Zot), deltaG, on the transport of a paracellular marker, mannitol, using in vitro (Caco-2 cell monolayers) and in vivo (intraduodenal administration in rats) experimental methods.

METHODS

The transport of [14C]mannitol with deltaG (0, 50, 80, or 100 microg/ml) was determined across Caco-2 cells. Male Sprague-Dawley rats were assigned to receive one of the following treatments: [14C] mannitol (40 microCi/kg), [14C]mannitol/deltaG (417 microg/kg), or [14C] mannitol/deltaG/Protease inhibitors (PI).

RESULTS

The mean (+/-S.E.M.) apparent mannitol permeability coefficients (P(app)) observed after incubation with 0, 50, 80, and 100 microg/ml deltaG were 3.5 (+/-0.4), 4.17 (+/-0.27), 4.33 (+/-0.61), and 9.94 (+/-0.24)x10(-6) cm/s. After oral administration, C(max) (3.8 x 10(-4) vs. 4.4 x 10(-4) mM) and AUC(0-6 h) (0.096 vs. 0.088 mM min), obtained for [14C]mannitol and [14C]mannitol/deltaG, respectively, were not statistically different. However, both C(max) (7.6 x 10(-4) mM) and AUC(0-6 h) (0.25 mM min) were significantly higher for the [14C]mannitol/deltaG/PI treatment.

CONCLUSIONS

The 12 kDa fragment of Zot, deltaG, enhanced the in vitro transport and oral absorption of the paracellular marker, mannitol, in the presence of protease inhibitors (PI).

Aluminum affects membrane physical properties in human neuroblastoma (IMR-32) cells both before and after differentiation

The capacity of Al(3+) to induce changes in the physical properties of plasma membrane from human neuroblastoma cells (IMR-32) was investigated, and the magnitude of the changes was compared with that obtained after cell differentiation to a neuronal phenotype. Similarly to our previous results in liposomes, Al(3+) (10 to 100 microM) caused a significant loss of membrane fluidity, being the differentiated cells more affected than the nondifferentiated cells. Al(3+) also increased the relative content of lipids in gel phase and promoted lipid rearrangement through lateral phase separation, with the magnitude of this effect being similar in nondifferentiated and differentiated cells. Since membrane physical properties depend on bilayer composition, we characterized the content of proteins, phospholipids, cholesterol, and fatty acids in the IMR-32 cells before and after differentiation. Differentiated cells had a significantly higher content of unsaturated fatty acids, creating an environment that favors Al(3+)-mediated effects on the bilayer fluidity. The neurotoxic effects of Al(3+) may be, at least in part, due to alterations of neuronal membrane physical properties, with potential consequences on the normal functioning of membrane-related cellular processes.

Interaction of aluminum with exogenous and endogenous iron in the organism of rats

The aim of these experiments was to find changes in free erythrocyte protoporphyrins (FEP) and in the concentration of endogenous iron in the blood, erythrocytes, serum, liver, kidneys, and spleen of rats, as well as in the dynamics of aluminum concentrations in the serum of rats after oral application of aluminum chloride (AlCl(3)) separately or with ferrum chloride (FeCl(2)), depending on the time and doses administered. The experiments were carried out on female Wistar rats which received (p.o.) 100 mg Al/kg separately or with iron (4 mg Fe/kg) daily for 35 days. The effects of aluminum administration were noticed after the second week. The experiments demonstrated that the increase in the level of free erythrocyte protoporphyrins in the blood is the most sensitive indicator of exposure to AlCl(3). A decrease in iron concentration in erythrocytes, blood, and spleen was also noticed. The response and the sequence of the investigated effects were recorded according to aluminum and iron concentration in the serum. Joint administration of iron and aluminum decreases concentration of aluminum in serum and prevents changes in the investigated indicators in rats exposed to aluminum chloride.

Al and Si: their speciation, distribution, and toxicity

OBJECTIVES

In dialysis patients both aluminum (AI) and silicon (Si) may accumulate. Whereas the toxic effects of AI within this population are clearly established, little is known on the role of Si in the development/protection of particular dialysis-related diseases. A clear insight in the protein binding and speciation of trace elements is important to better understand the mechanisms underlying their toxicity/essentiality. Research in this field however is complex and often prone to analytical difficulties and inaccuracies.

DESIGN AND METHODS

In the first part of this review techniques used for speciation studies of AI and Si in biological fluids are discussed. Notwithstanding recent technical advances (a) extraneous metal contamination, (b) unrecognized aspecific binding of metals to proteins, and (c) unwanted interactions with separation equipment such as chromatography columns and ultrafiltration membranes remain important pitfalls and often lead to erroneous conclusions. The factors that determine the speciation of AI and Si and their ultimate tissue distribution and toxicity are dealt with in the second part. Here, experimental data obtained with various speciation techniques are linked to in vivo data on the tissue distribution, localization/toxicity of both elements.

CONCLUSIONS

A model in which the AI tissue distribution/toxicity is mediated by either its citrate or transferrin bound form is proposed.