Zinc complexes albumin

Some of the zinc taken up by the intestinal epithelial cells is rapidly transferred to the portal plasma where it associates with albumin, a2 macroglobulin, and amino acids. About 67% of the zinc in plasma is bound to albumin, and about 3% is stored in liver (Stemlieb 1988). Soluble organozinc complexes are passively absorbed across the plasma membrane of the mucosa of the intestinal villi the soluble, nondiffusable complexes are transported in the intestinal products and excreted in feces (NAS 1979). Zinc loss from urine and sweat is usually small (Casey and Hambidge... 

In a normal human adult, about 2 g of zinc is filtered by the kidneys daily and about 0.3 to 0.6 mg is actually excreted each day (Goyer 1986). Zinc homeostasis in rats, unlike most mammals, is maintained by zinc secretion from the intestines rather than by regulation of zinc absorption (Elinder 1986). Initial uptake of zinc from the rat gastrointestinal tract involves binding to albumin and transport of the zinc-albumin complex from intestine to liver (Hoadley and Cousins 1988). 

In addition to factors Influencing luminal uptake of zinc, transfer across the basolateral membrane has been shown to be dependent on the concentration of albumin in the portal circulation (33). These investigations suggest that metabolic factors which affect the albumin concentration in the plasma may also affect the rate of portal zinc transfer. It should be noted that EDTA did not enhance zinc accumulation within the mucosal cells yet it Increased transfer to the vascular perfusate. These results suggest that basolateral membrane transport of zinc is enhanced by EDTA. We have proposed (35), as has Davies (38), that basolateral transport to the circulation is the rate limiting phase of zinc absorption. Since EDTA and zinc might be transported as a complex (42), the latter may transverse this barrier more easily and thus Increase zinc absorption. 

Labile proteins. These include proteins such as transferrin (which complexes iron, as discussed previously) and albumin (which complexes copper, zinc, and other metals). 

It is noteworthy that addition of another ligand (which can be histidine or cysteine) to carnosine complexes cause increased stability [30]. Addition of albumin leads to the similar effect [28], thus carnosine can effectively compete with albumin for copper or zinc ions [29],... 

The observations of Klotz and Loh-Ming (1954) on the binding of pyri-dine-2-azo-dimethylaniline to protein through a cation bridge indicate that a similar behavior of zinc occurs in mixed protein-small molecule complexes. They have measured the stability constants of the dye with cations bound to protein (pepsin and albumin) and free in solution. For Zn and Cu specifically ... 

Zinc is excreted from the body primarily in the faeces. There is a substantial en-terohepatic re-circulation, and as Cousins (1982) notes, the gut gets two zinc meals per day, one from foodstuff, the other from zinc re-entering the intestinal lumen in digestive juices and crossing the mucosal barrier from the plasma zinc-albumin complex. The total zinc output in faeces usually equals the dietary intake at around 10-15 mg per day. 

Demonstration of Peptonizing Power Method with Edestin. — The peptonizing power of pepsin is exerted much less quickly than the dissolving power. Under the action of pepsin, a progressive hydrolysis is effected which results in the formation of acid albumins and albumoses, bodies which are precipitated by saturation of the liquor with zinc sulphate or ammonium sulphate, and complex polypeptides. The chemical transformations which albuminoid matter imdergoes imder the influence of pepsin will constitute the subject of another chapter. 

In the nondiffusible form, a small amount of circulating zinc is tightly bound to a -Macroglobulin in the plasma (Cousins 1985). Zinc is incorporated into and dissociated from oe -macroglobulin only in the liver (Henkin 1974). This zinc-protein complex has an association constant of >10 (Henkin 1974 NAS/NRC 1979). The zinc bound to a2-Macroglobulin is not freely exchangeable with other zinc ligands (i.e., zinc-albumin and zinc-amino acid complexes) in serum. 

The Job method has been used to show that horse-liver alcohol dehydrogenase contains zinc atoms bound in two different environments, and it appears that Zn + plays a functional role in the interaction of DNA with DNA polymerases from E. coli and sea urchins. The nature of the two metal-binding sites in transferrin has been investigated, using tervalent lanthanide ions as fluorescent probes the two sites are different and the distance between them is 43 A or greater. The kinetics of the reaction between Fe -nta and transferrin have been measured by the stopped-flow method and a reaction sequence has been proposed. The same technique, with C1 n.m.r. detection, has been used to study the extraction of the metal from the Hg -bovine serum albumin complex by various ligands. 

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