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Specific binding proteins for selenium in rat tissues
B P Sani1, J L Woodard, M C Pierson
1Kettering-Meyer Laboratory, Southern Research Institute, Birmingham, AL 35255-5305.
Abstract:
The preventive and therapeutic potential of selenium (Se), a micronutrient, against cancer has been well documented in several test systems, but the mechanism of its action is not known. The possibility that Se might function in a manner similar to steroid hormones and retinoids through mediation of cellular receptors was examined. A specific 2S cellular binding protein (SeBP) for Na2[75Se]O3 was detected in rat tissue extracts. Liver and intestine exhibited highest levels of SeBP, and heart, uterus and spleen had the lowest levels. Oral administration of Na2[75Se]O3 to rats resulted in its uptake by the tissues with concomitant appearance of [75Se]SeBP complex. The protein binds sodium selenite with moderately high affinity; the apparent dissociation constant was determined by Scatchard analysis to be 1.1 X 10(-7) M. SeBP focused at pH 5.3 upon isoelectric focusing in ampholines of pH 3-10. Competitive binding affinity studies with unlabeled test compounds revealed that selenium dioxide and selenocystine showed high binding affinity (90-95%) for the selenite-binding site on SeBP. Sodium selenate, elemental Se powder, and selenomethionine, however, showed poor competition with sodium selenite. Biological activity of the above selenocompounds, as expressed by others, correlate with their binding affinities for SeBP. Sodium sulfite showed 35% inhibition of Na2[75Se]O3 binding, but sulfate showed none. Two ultimate carcinogens, N-methyl-N-nitrosourea and N-methyl-N'-nitro-N-nitrosoguanidine, and two retinoids, retinol and retinoic acid, showed less than 10% inhibition of binding. Interaction of Se with SeBP is completely blocked by thiol inhibitors. Plasma transport of Na2[75Se]O3 is mediated by a protein with a mol. wt of 68,000, which is presently identified, by immunoprecipitation studies as well as by Affi-Gel Blue column chromatographic experiments, as serum albumin. The results suggest that the plasma transport of Se is facilitated by albumin, and that the intracellular transport of Se for its biological functions is accomplished by SeBP.
Insights
Selenium (Se) acts like a hormone, binding to a specific cellular protein (SeBP) in tissues like the liver and intestine. This SeBP facilitates selenium
Area of Science:
- Biochemistry
- Molecular Biology
- Nutritional Science
Background:
- Selenium (Se) is a vital micronutrient with documented anticancer properties.
- The precise mechanisms underlying selenium's biological actions remain largely unknown.
- Investigating potential receptor-mediated pathways, similar to steroid hormones and retinoids.
Purpose of the Study:
- To elucidate the mechanism of selenium's action by identifying cellular receptors.
- To characterize a specific selenium-binding protein (SeBP) and its role in selenium transport and function.
- To explore the binding affinities of various selenium compounds to SeBP.
Main Methods:
- Detection and characterization of a 2S cellular binding protein (SeBP) for Na2[75Se]O3 in rat tissues.
- Assessment of SeBP levels in different organs (liver, intestine, heart, uterus, spleen).
- Competitive binding assays using various unlabeled selenium compounds and other molecules.
- Scatchard analysis to determine binding affinity (dissociation constant).
- Isoelectric focusing for protein characterization.
- Identification of plasma transport protein using immunoprecipitation and chromatography.
Main Results:
- A specific SeBP was identified in rat tissues, with highest concentrations in liver and intestine.
- Oral administration of Na2[75Se]O3 led to the formation of a [75Se]SeBP complex in tissues.
- SeBP exhibits moderate-high affinity for sodium selenite (Kd = 1.1 x 10(-7) M).
- Selenium dioxide and selenocystine demonstrated high binding affinity, while selenomethionine showed poor affinity.
- Biological activity of selenocompounds correlated with their SeBP binding affinities.
- Thiol inhibitors blocked Se interaction with SeBP.
- Serum albumin was identified as the primary plasma transport protein for selenium.
Conclusions:
- Selenium's biological functions may be mediated by a specific intracellular binding protein (SeBP).
- SeBP plays a crucial role in the intracellular transport and potential action of selenium.
- Plasma transport of selenium is facilitated by serum albumin, while intracellular transport involves SeBP.