Chromium biological function

The behavior of metals as atoms or ions deeply affects the electrochemical reactions they undergo, and similarly affects the metabolism of plants and animals. Iron, copper, cobalt, potassium, and sodium are examples of metals that are essential to biological function. Some metals such as cadmium, mercury, lead, barium, chromium, and beryllium are highly toxic. 

A description of the most important properties of elements of interest, because they are essential for (or toxic to) life, is given in this section. A preliminary outline of biological function and level of toxicity is provided in Table 9.1. Those elements listed in the table that are more toxic to animals than to plants present the most insidious hazard to human health because the elements may accumulate in apparently healthy plants to levels that are poisonous to humans and animals. That is, for such elements the plant fails to play the role of a biological alarm that would warn of toxicity. The rating of elemental phytotoxidty in Table 9.1 should not be confused with the actual likelihood or frequency of toxicity in the field. For example, manganese is rated to have a fairly low intrinsic toxicity to plants, yet its toxicity is commonly seen because very high Mn concentrations can develop in wet soils. In contrast, chromium and lead are rated as more phytotoxic, but they are generally so insoluble in soils that toxicity from these metals is rarely seen. 

The first-row transition metals from chromium through zinc all have some biologic function in the human body. How many unpaired electrons are present in each of these first-row transition metals in the ground state ... 

The toxicity of chromium is profoundly related to its oxidation state, whether the metal is in the +3 or +6 oxidation state. Chromium is a naturally occurring metal that is widely used for industrial purposes including plating, leather tanning, as a dye and as a wood preservative. Trivalent chromium (Cr ) is an essential trace nutrient required for proper glucose metabolism and other biological functions. 

Some metals, including heavy metals, affect negatively people s health, either individually or forming metal compounds, and therefore they must be controlled. Moreover, there are toxic semi-metaUic elements the so-called metalloids, such as arsenic and selenium, which should also be monitored. Basically, this group of pollutants can be classified in (1) metals and metalloids that are necessary to support life in very small amounts, but toxic in larger amounts, e.g. copper, zinc, chromium, arsenic, or selenium (2) metals with unknown biological function which are extremely toxic even at trace levels, e.g. mercury, cadmium, or lead. 

Many of the most important catalytic activities of coordination compounds and metal ions (particularly iron and copper) are in the electron transport chains of cellular metabolism, where they act as catalysts for the oxidation of organic intermediates. Several other transition metal ions (including vanadium and molybdenum) have important metabolic roles in a variety of organisms. Indeed, recent discoveries suggest that even such metals as chromium and nickel have biological functions. 

The quality of the experimental evidence for nutritional essentiality varies widely for the ultratrace elements. The evidence for the essentiality of three elements, iodine, molybdenum and selenium, is substantial and noncontroversial specific biochemical functions have been defined for these elements. The nutritional importance of iodine and selenium are such that they have separate entries in this encyclopedia. Molybdenum, however, is given very little nutritional attention, apparently because a deficiency of this element has not been unequivocally identified in humans other than individuals nourished by total parenteral nutrition or with genetic defects causing disturbances in metabolic pathways involving this element. Specific biochemical functions have not been defined for the other 15 ultratrace elements listed above. Thus, their essentiality is based on circumstantial evidence, which most often is that a dietary deprivation in an animal model results in a suboptimal biological function that is preventable or reversible by an intake of physiological amounts of the element in question. Often the circumstantial evidence includes an identified essential function in a lower form of life, and biochemical actions consistent with a biological role or beneficial action in humans. The circumstantial evidence for essentiality is substantial for arsenic, boron, chromium, nickel, silicon, and vanadium. The evidence for essentiality for the... 

The very important role of the heme system, Fe(Proto)LL ( 3) in biological oxygen transport and consumption as well as electron transport is a main topic not only of biochemists, but of bioinorganic chemists and biomimetic chemists as well for this general topic, the reader may consult some recent review articles [14-21,22]. Bioinorganic chemists have studied the effect of replacement of iron by other 3d metals, especially chromium, manganese, and cobalt, and frequently, interesting structural, spectral, or functional models [14,20] of the heme enzymes have been found with these metals. 

Chromium(III) is an essential nutrient required for normal energy metabolism. The National Research Council (NRC) recommends a dietary intake of 50-200 ig/day (NRC 1989). The biologically active form of an organic chromium(ni) complex, often referred to as GTF, is believed to function by facilitating the interaction of insulin with its cellular receptor sites. The exact mechanism of this interaction is not known (Anderson 1981 Evans 1989). Studies have shown that chromium supplementation in deficient and marginally deficient subjects can result in improved glucose, protein, and lipid metabolism. 

MertzW. 1969. Chromium occurrence and function in biological systems. Physiol Rev 49(2) 163-239. 

---