Rare earth element in human organs and tissues

Formation of technogenic geochemical provinces in modern conditions has become a cause of serious concern due to the negative consequences of polyelement impact of the contaminated environment on living organisms, including a man (Gichev, 2002; Revich, 2001). Such influence is often connected with the operation of certain enterprises and characterizes a local situation.

As early as in the thirties of the XX century academician V.I. Vernadsky wrote that entire D.I. Mendeleev’s table is included in human organism. But, despite great discoveries in chemistry, in the field of biogeochemistry the results are not significant. Element structure of an organism has been poorly studied. Therefore, in this case V.I. Vernadsky’s words are quite appropriate: “At present the main disadvantage is absence of complete, element chemical quantitative analysis in living substance. Now we have no such data compared to, foe example, the analysis of minerals or the newest analysis of rocks for an organism. Nether have them, for example, for such an organism as a human being, whose organism has already been studied for centuries through persistent work of scientists, who have developed a huge number of independent scientific disciplines, for example, biogeochemistry – a science about element structure of living substance. Thus, in terms of demography and anthropology we have relatively accurate data on substance weight making homogeneous living substance related to a man. However, the values of his average chemical composition are presented” (Vernadskiy, 1960).

At present (XXI century) the problem of accurate quantitative element structure of human organism is still rather topical. The most complete report of human chemical composition for today, widely used by specialists, is the report of working group II of WHO in conditional man (“Human..”, 1977), where the information on composition of 71 organs and tissues of 150 adults died as a result of accidents and obtained by common analysis method is included as well as the data from other sources are generalized which are often recalculated as they are expressed in different units of weight: milliequivalent – percent, milligram – percent, microgram – per gram and so on and characterize crude or dry tissue, ash etc. In this report the data on 47 chemical elements are presented including natural radioactive elements, but the information on rare-earth elements (REE) content in human organism is not available. Some information on the given group of elements in organs and tissues of a man is in J. Emsly’s book “Elements” (1993). All studies of human element composition are complicated by the problem of obtaining bioptic material because of ethic norms.

In the course of our research we have chosen a group of REE as a least studied group of chemical elements in living organism. Rare earth elements are a group of osteotropic elements (Skoblin, 1968). When entering in organism REE take part in mineral metabolism and quickly concentrate in inorganic part of bone tissue (Balabuha, 1958, 1962). But they influence not only human bone tissue but also accumulate in breath, digestion, endocrine, and urinogenital systems. Accumulation of elements in definite tissues depends directly on blood supply of the bodies possessing the capability to keep more or less blood. The ways of entering these elements into organism are also of definite significance. Once the rare earth elements enter the organism, they are held back for a long period in it. In this case the processes of accumulation and distribution in organs and tissues and their slow excretion depend on the course and character of salt hydrolysis reaction of rare earth elements to the great extent. Salt hydrolysis results in formation of insoluble colloidal hydroxides which are adsorbed by tissues and held back in the organism (Balabuha, 1962; Shtreffer, 1972). In the literature there is information about a number of experiments in introduction and excretion of radioactive and rare earth elements by the example of some animals: rats, mice, dogs, rabbits, but despite availability of such interesting data there is no quantitative evaluation of studied element concentration in organs and tissues. REE are a group of poorly examined elements, the information on their biological role, concentration in living organisms or biosubstrates being limited or absent.

For this reason in the given investigation we focus on the processes of accumulation and distribution of rare earth elements in human organs and tissues. Examination of migration, distribution and concentration of rare earth elements is impossible without having idea of relation between chemical, physical-chemical properties of these elements and biochemical behavior of the organism.

Thus, we studied human organs and tissues in concentration of 56 chemical elements including data obtained for the first time on accurate content of rare earth elements (REE) group and radioactive elements in human organism in Tomsk region.

The distribution of all known 14 REE in human organs and tissues was considered. According to the geochemical rules and laws all REE were divided into three groups in weight and the most striking examples were distinguished: 1. light REE – lanthanum and cerium; 2. middle REE – samarium and europium; 3. heavy REE – ytterbium and lutetium (Fig. 1).

Having considered and analyzed Fig. 1, one can say that content and distribution of the given three groups of REE is rather logical. On the first group of REE the concentration of cerium is higher than that of lanthanum in all organs except heart where concentration of lanthanum is even higher; in large intestine and mammary gland the content is the same. The similar trends in concentrations were observed in spleen, trachea, bronchi, and lungs. It should be noted that the maximum concentrations of cerium (0,13 mg/kg) and lanthanum (0,054 mg/kg) were found in lungs. In the second group of REE the tendencies are though the same, but there are some abnormalities, for example, in esophagus, heart, aorta the concentrations of europium is higher than that of samarium, which is not appropriate. The given fact can be explained by the chemical form of entry and migration of europium in human organism. In duodenum, thyroid gland, and mammary gland the concentrations of samarium and europium coincide. In the breath system in this group samarium in bronchi (0,0032 mg/kg) and europium in trachea (0,0009 mg/kg) prevail. In the third group there is a common tendency in concentration of ytterbium and lutetium, in esophagus approximately the same concentration of the latter elements is observed. The maximum content of ytterbium and lutetium is typical for bronchi, 0,0014 and 0,00023 mg/kg respectively.

Fig. 1. Distribution of rare earth elements in human organs and
tissue (mg/kg)

1 – tongue; 2 – esophagus; 3 – stomach; 4 – duodenum; 5 – small intestine; 6 – large intestine; 7 – liver; 8 – pancreas; 9 – cava; 10 – heart; 11 – aorta; 12 – spleen; 13 – trachea; 14 – bronchi; 15 – lung; 16 – thyroid gland; 17 – adrenals; 18 – bladder; 19 – kidney; 20 – brain; 21 – skeleton muscular system; 22 – skin; 23 – adipose tissue; 24 - mammary gland; 25 – ovary; 26 – uterus.

One can give explanation of concentrations of rare earth elements in organs and tissue of Tomsk region inhabitants in terms of geochemical features of Tomsk oblast. On its territory there are large industrial enterprises, among which the most important are nuclear thermal center, Siberian chemical combine, as well as deposits of brown coal, zircon-ilmenite sand (Rikhvanov et al., 2006; 2008). All these factors influence the entry of the investigated group of elements into human organism. But they are not all the arguments. To determine the behavior of rare earth elements in human organism completely it is necessary to consider human organism itself in more details. For instance, having plotted a graph (Fig.1) on the REE content in human organs and tissues some interesting facts, that can be explained in terms of biochemical properties of a man’s organism, have been revealed.

One of the features of REE behavior is their tendency to adsorption. This property is explained by the fact that surface of any particle, even a dust particle, is comparatively large for the number of REE. In living organisms the conditions of adsorption and precipitation are particularly favorable (large surface of protein molecules). REE capability for adsorption is not the same everywhere and depends on solubility degree of their compounds. The higher is solubility, the less is their capability for adsorption, and vice versa (Balabucha, 1962). The group of REE is particular disposed to adsorption and precipitation, because their salt and base solubility is rather insignificant, for example, the product of solubility of La(OH)₃ is 1,0*10^-20. REE act in the organism according to their chemical properties. Hence, for example, they form complex compounds. The element behavior in organism is strongly influenced by the form of compound, with which they are connected in organism. As physical-chemical conditions in different environments vary, the forms of their compounds can be different too. Coming up against different chemical compounds in tissue structures or products of their metabolism, rare earth elements enter into exchange reactions. Salts of rare earth elements (ytterbium, cerium and other lanthanides) change into insoluble hydroxides at pH equal to 6,8-8,4: рН (La)=8,4; рН (Ce)=7,4 рН (Pr)=7,1 рН (Nd)=7,0; рН (Sm)=6,8 (Balabucha, 1962). Such nearly neutral alkaline conditions are typical for the following organs and tissues of a man: saliva – mostly alkaline reaction (fluctuations of рН 6,0-7,9) [4], liver – the reaction of cystic bile is close to neutral (рН is approximately 7,0), the reaction of hepatic bile is alkaline (рН 7,5 - 8,0), pancreatic gland – pancreatic juice is alkalescent, small intestine – alkaline reaction, large intestine – subacid reaction. Hence, rare earth elements are turned into sparingly soluble compounds just in these organs and tissues.

In general, having studied the REE distribution in organs and tissues one can conclude that when entering human organism REE are involved into a number of complex reactions. Having analyzed the results obtained, it was revealed that accumulation of REE spectrum occurs in female ovary, spleen, and adrenal gland, and in male skin, bronchi, and lungs. It should be noted that maximum concentrations of studied elemental group are typical, first of all, for organs of breath system that can be explained by the rare earth element capability to form complexes and precipitate directly in the given system, as this system is like a sponge where elements including rare earth elements are accumulated. It is also of particular importance that smoking is a factor provoking and contributing to REE concentration in breath system. As to distribution of definite elements one can say that erbium was isolated, for male thyroid gland its maximum concentration was stated with respect to the other REE. The given feature can cause possible dysfunction of male thyroid gland, and in female the given pathology is connected with excess of neodymium. Male has dysfunctions of digestive system. On the one hand these properties can be explained by physiological conditions, on the other hand, by the forms of element occurrence. Hence, for example, europium can exist in two and trivalent state, in this case in the trivalent form it is a sparingly soluble compound. Therefore, in pancreatic gland where pancreatic juice is secreted which is a transparent watery liquid with pronounced alkaline properties (рН=8,5), europium forms poorly soluble and less movable compounds resulting in its elevated concentrations. REE accumulation in definite organs depends on blood supply of organs having capability to keep more or less quantity of blood. That is why the increased concentration of REE in female spleen is explained by the fact that the given organ is a «blood depot».

Studying the distribution of rare earth elements (REE) the organs were distinguished in which the law of decrease in element distribution and its concentration with growth of nuclear charge as well as Oddo-Garkin’s law of even and uneven elements is observed. These regularities are clearly observed in the following organs: spleen, bronchi, lungs, liver, ovary, adipose tissue; there are the organs where the law is broken in one element: brain, mammary gland, adrenal gland, skeleton muscular system, kidneys, bladder, heart, cava, trachea, stomach, large intestine; in two elements: pancreatic gland, thyroid gland, skin, aorta, esophagus, duodenum, small intestine.

Thus, REE distribution in organs and tissues is rather irregular and their difference in concentration can amount several orders (Ignatova, 2008). On the whole, accumulation of rare earth elements in human organs and tissues follows the general geochemical laws of Klark and Oddo-Garkins of chemical element distribution in the universe. The level of REE accumulation in human organism is determined by not only biochemical and biophysical features of living tissue functions but also sex and presumably age conditions, pathological transformations as well as factors of natural environment of a man and state of these elements.

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