Title: Chromium
Key words: chromium, micronutrient, carbohydrate, lipid, metabolism, insulin, receptors, analysis, chromodulin, glucose, diabetes, glucose tolerance, factor,
Date: Oct 2006
Category: Micronutrients
Nutrimed Module:
Type: Article
Author: Morgan, G
Chromium
Chromium has been shown to be an essential micronutrient involved in normal carbohydrate and lipid metabolism (Anderson 1985). Research has focused on its role in carbohydrate metabolism and, recently, the interactions of the biologically active form of chromium with insulin receptors has become an important area of research. The following relate to this area of research:
a) Chromium analysis
Problems of background contamination have bedevilled accurate analysis of the very low levels of chromium in the body. Veillon in 1989 reported that plasma and serum chromium was virtually undetectable with the then current instrumentation. In 1999 he reported that current instrumentation was now able to detect levels of chromium some 1000 times less than a decade or so before (Veillon 1999). This increase in sensitivity has in large measure been due to more sophisticated facilities for correcting background contamination. Earlier systems of atomic absorption spectroscopy using deuterium have been replaced with tungsten-halogen correctors with vastly improved sensitivity. The latest systems using a Zeeman-effect background correction are even more sensitive.
Sample collection, storage, preparation and analysis remain critical in working with chromium samples. Contamination is easily introduced: the use of specialised plastic containers, freezing to control water vaporization, the application of strict protocols and the implementation of the strictest in-house quality control procedures by specialised laboratories is essential if valid results are to be achieved.
b) Low molecular weight
chromium binding substance (LMWCr)
Research has identified the presence of a low molecular weight oligopeptide which binds chromium present in the blood in a
transferring form enabling insulin receptors on cell surfaces to be activated
(Yamamoto 1989, Vincent 1999). The oligopeptide
contains only the four amino acids glycine, cysteine, glutamate and aspartate,
has a molecular weight of around 1500 and is thought to bind four Cr3+ ions.
Binding results in the inactive apo- form being
changed into the active LMWCr or ‘chromodulin’
form. It is this form which is able to bind with insulin/insulin receptor sites
to form complexes which initiate a series of kinase-mediated
cascades associated with glucose metabolism (Davis 1996, Davis 1997). LMWCr not only fulfils a key role in glucose metabolism but
the oligopeptide also binds to chromium in this form
and is excreted in the bile and urine (Wada 1983, Manzo
1983). As such it may be implicated in the increased urinary excretion of
chromium and the possible chromium- mediated renal pathology of diabetes
mellitus (Morris 1992). Further research is needed in this area.
c) Glucose tolerance factor
The
term glucose tolerance factor (GTF) was coined by Schwarz and Mertz (1957) to
describe a substance which was able to reverse the biochemical changes
associated with impaired glucose tolerance. Two forms have been described, one
from acid-hydrolysed porcine kidney powder and the other from Brewer’s yeast.
The latter has been extensively studied so that GTF usually refers exclusively
to this form of bound chromium. Due to its increased biological activity it was
thought that the chromium bound in GTF owed its insulin-binding activity to the
presence of a bioactive macromolecule. Analysis of Brewer’s yeast GTF showed it
to be a complex of chromium, nicotinic acid, glycine,
glutamic acid and cysteine
but, contrary to expectations its glucose lowering activity appears to be
diminished rather than potentiated in the presence of
insulin (Vincent 1994). Vincent also commented upon the relationship between
GTF and LMWCr (Vincent 1994). It is now thought that
GTF simply represents a more readily bioavailable
source of chromium. It is of some interest as the picolinate
form of chromium, now widely used as a supplement, was developed as a
by-product of research into chromium-nicotinate
complexes.
References
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