9 1992 by The Humana Press, Inc. All rights of any nature, whatsoever, reserved. 0163-4984/92/3301-3-~151 $02.00
Some Properties of Selenoprotein P RAYMOND F. BURK AND KRISTINA E. HILL Division of Gastroenterology, Department of Medicine and Center in Molecular Toxicology, Vanderbilt University School of Medicine, Nashville, TN Received August 29, 1991; Accepted October 11, 1991
ABSTRACT Selenoprotein P is a newly characterized selenoprotein. It is the first protein described to contain multiple selenocysteines. It is secreted by the liver into the plasma and turns over rapidly. Its concentration is sensitive to the selenium status of the animal. Its function is unknown. Index Entries: Selenium; Selenoprotein P.
INTRODUCTION Selenium has biochemical effects that cannot be explained by the selenoenzyme glutathione peroxidase. This has led to a search for other selenoproteins. In 1977, Herrman (1) identified a rat plasma protein containing selenium that was distinct from glutathione peroxidase. Studies by two other groups followed in 1982 (2,3) showing that a plasma protein other than glutathione peroxidase incorporated 75dSe rapidly after its injection as selenite. Because of its plasma location, the protein was designated selenoprotein P. Efforts to purify selenoprotein P by conventional means were unsuccessful and only after immunoaffinity chromatography was applied could it be purified (4). Subsequent studies have characterized the protein (5), determined the effect of dietary selenium on it (6), evaluated its physiology (7), and cloned and sequenced its cDNA (8).
*Author to whom all correspondence and reprint requests should be addressed. Biological Trace Element Research
]5]
Vol. 33, 1992
152
Burk and Hill
CHARACTERISTICS OF SELENOPROTEIN P Analysis of the cDNA sequence of selenoprotein P predicts a 366 amino acid polypeptide of 41,052 daltons (8). A 19 amino acid signal sequence is present consistent with its secretion by the liver. The protein is glycosylated and migrates at 57 kDa on SDS/PAGE in the reduced state (4,5). After N-deglycosylation, it migrates at 43 kDa. The protein is rich in cysteine (17 residues) and histidine (28 residues), and the cDNA contains 10 TGAs in the open reading frame. TGA codes for selenocysteine in other selenoproteins and presumably also in selenoprotein P. Nine of the 10 selenocysteines are located in the terminal one-third of the protein. This might indicate that this part of the protein has a redox function. Comparison of selenium content and selenoprotein P content in selenium-deficient serum reveals that the selenium content is too low to account for the selenoprotein P measured. This raises the possibility that there might be selenium-poor forms of selenoprotein P.
PHYSIOLOGY OF SELENOPROTEIN P Selenoprotein P contains about 65% of the selenium in rat plasma (5). Its concentration is 26 I~g protein/mL plasma. Protein synthesis is required for incorporation of selenium into selenoprotein P. The half-life has been estimated to be 3-4 h by studies in which semipurified selenoprotein P was injected intravenously (7). This indicates a turnover of 13 i~g/100 g . h, which is approx 3% of the turnover of albumin. This suggests that the synthesis rate of selenoprotein P is relatively high. Administration of selenium as selenoprotein P was carried out to test the hypothesis that selenoprotein P is a selenium transport protein. In that experiment, selenium-deficient brains took up more of the dose than did selenium-adequate ones (7). However, no such uptake was observed in other tissues. Therefore, with the possible exception of the brain, selenoprotein P does not appear to serve tissues as a selenium transporter. Its function remains unknown.
NUTRITIONAL EFFECTS The effect of dietary selenium on selenoprotein P concentration was studied using a radioimmunoassay. Levels of the selenoprotein were sensitive to selenium status and generally mirrored glutathione peroxidase activity (6). However, at levels of dietary selenium below the nutritional requirement, selenoprotein P concentrations were higher in proportion to control values than were glutathione peroxidase activities. This demonstrates that selenoprotein P has a greater call on limiting quantities of selenium than does glutathione peroxidase.
Biological Trace Element Research
Vol. 33, 1992
Some Properties of Selenoprotein P
153
Injection of selenium into selenium-deficient rats increased selenoprotein P levels within hours, in contrast to glutathione peroxidase activity, which rose much later. This is a further indication that selenoprotein P synthesis has a greater priority than that of glutathione peroxidase.
SUMMARY Selenoprotein P is a newly characterized selenoprotein. It is the first protein described to contain multiple selenocysteines. It is secreted by the liver into the plasma and turns over rapidly. Its concentration is sensitive to the selenium status of the animal. Its function is unknown.
REFERENCES 1. J. L. Herrman, Biochim. Biophys. Acta 500, 61-70 (1977). 2. R. F. Burk and P. E. Gregory, Arch. Biochem. Biophys. 213, 73-80 (1982). 3. M. A. Motsenbocker and A. L. Tappel, Biochim. Biophys. Acta 719, 147-153 (1982). 4. J. -G. Yang, J. Morrison-Plummer, and R. F. Burk, J. Biol. Chem. 262, 13,372-13,375 (1987). 5. R. Read, T. Bellew, J. -G. Yang, K. E. Hill, I. S. Palmer, and R. F. Burk, J. Biol. Chem. 265, 17,899-17,905 (1990). 6. J. -G. Yang, K. E. Hill, and R. F. Burk, J. Nutr. 119, 1010-1012 (1989). 7. R. F. Burk, K. E. Hill, R. Read, and T. Bellew, Am. J. Physiol. 261, E26-E30, (1991). 8. K. E. Hill, R. S. Lloyd, J. -G. Yang, R. Read, and R. F. Burk. ]. Biol. Chem. 266, 10050-10053 (1991).
Biological Trace Element Research
Vol...33, 1992
Some Properties of Selenoprotein P RAYMOND F. BURK AND KRISTINA E. HILL Division of Gastroenterology, Department of Medicine and Center in Molecular Toxicology, Vanderbilt University School of Medicine, Nashville, TN Received August 29, 1991; Accepted October 11, 1991
ABSTRACT Selenoprotein P is a newly characterized selenoprotein. It is the first protein described to contain multiple selenocysteines. It is secreted by the liver into the plasma and turns over rapidly. Its concentration is sensitive to the selenium status of the animal. Its function is unknown. Index Entries: Selenium; Selenoprotein P.
INTRODUCTION Selenium has biochemical effects that cannot be explained by the selenoenzyme glutathione peroxidase. This has led to a search for other selenoproteins. In 1977, Herrman (1) identified a rat plasma protein containing selenium that was distinct from glutathione peroxidase. Studies by two other groups followed in 1982 (2,3) showing that a plasma protein other than glutathione peroxidase incorporated 75dSe rapidly after its injection as selenite. Because of its plasma location, the protein was designated selenoprotein P. Efforts to purify selenoprotein P by conventional means were unsuccessful and only after immunoaffinity chromatography was applied could it be purified (4). Subsequent studies have characterized the protein (5), determined the effect of dietary selenium on it (6), evaluated its physiology (7), and cloned and sequenced its cDNA (8).
*Author to whom all correspondence and reprint requests should be addressed. Biological Trace Element Research
]5]
Vol. 33, 1992
152
Burk and Hill
CHARACTERISTICS OF SELENOPROTEIN P Analysis of the cDNA sequence of selenoprotein P predicts a 366 amino acid polypeptide of 41,052 daltons (8). A 19 amino acid signal sequence is present consistent with its secretion by the liver. The protein is glycosylated and migrates at 57 kDa on SDS/PAGE in the reduced state (4,5). After N-deglycosylation, it migrates at 43 kDa. The protein is rich in cysteine (17 residues) and histidine (28 residues), and the cDNA contains 10 TGAs in the open reading frame. TGA codes for selenocysteine in other selenoproteins and presumably also in selenoprotein P. Nine of the 10 selenocysteines are located in the terminal one-third of the protein. This might indicate that this part of the protein has a redox function. Comparison of selenium content and selenoprotein P content in selenium-deficient serum reveals that the selenium content is too low to account for the selenoprotein P measured. This raises the possibility that there might be selenium-poor forms of selenoprotein P.
PHYSIOLOGY OF SELENOPROTEIN P Selenoprotein P contains about 65% of the selenium in rat plasma (5). Its concentration is 26 I~g protein/mL plasma. Protein synthesis is required for incorporation of selenium into selenoprotein P. The half-life has been estimated to be 3-4 h by studies in which semipurified selenoprotein P was injected intravenously (7). This indicates a turnover of 13 i~g/100 g . h, which is approx 3% of the turnover of albumin. This suggests that the synthesis rate of selenoprotein P is relatively high. Administration of selenium as selenoprotein P was carried out to test the hypothesis that selenoprotein P is a selenium transport protein. In that experiment, selenium-deficient brains took up more of the dose than did selenium-adequate ones (7). However, no such uptake was observed in other tissues. Therefore, with the possible exception of the brain, selenoprotein P does not appear to serve tissues as a selenium transporter. Its function remains unknown.
NUTRITIONAL EFFECTS The effect of dietary selenium on selenoprotein P concentration was studied using a radioimmunoassay. Levels of the selenoprotein were sensitive to selenium status and generally mirrored glutathione peroxidase activity (6). However, at levels of dietary selenium below the nutritional requirement, selenoprotein P concentrations were higher in proportion to control values than were glutathione peroxidase activities. This demonstrates that selenoprotein P has a greater call on limiting quantities of selenium than does glutathione peroxidase.
Biological Trace Element Research
Vol. 33, 1992
Some Properties of Selenoprotein P
153
Injection of selenium into selenium-deficient rats increased selenoprotein P levels within hours, in contrast to glutathione peroxidase activity, which rose much later. This is a further indication that selenoprotein P synthesis has a greater priority than that of glutathione peroxidase.
SUMMARY Selenoprotein P is a newly characterized selenoprotein. It is the first protein described to contain multiple selenocysteines. It is secreted by the liver into the plasma and turns over rapidly. Its concentration is sensitive to the selenium status of the animal. Its function is unknown.
REFERENCES 1. J. L. Herrman, Biochim. Biophys. Acta 500, 61-70 (1977). 2. R. F. Burk and P. E. Gregory, Arch. Biochem. Biophys. 213, 73-80 (1982). 3. M. A. Motsenbocker and A. L. Tappel, Biochim. Biophys. Acta 719, 147-153 (1982). 4. J. -G. Yang, J. Morrison-Plummer, and R. F. Burk, J. Biol. Chem. 262, 13,372-13,375 (1987). 5. R. Read, T. Bellew, J. -G. Yang, K. E. Hill, I. S. Palmer, and R. F. Burk, J. Biol. Chem. 265, 17,899-17,905 (1990). 6. J. -G. Yang, K. E. Hill, and R. F. Burk, J. Nutr. 119, 1010-1012 (1989). 7. R. F. Burk, K. E. Hill, R. Read, and T. Bellew, Am. J. Physiol. 261, E26-E30, (1991). 8. K. E. Hill, R. S. Lloyd, J. -G. Yang, R. Read, and R. F. Burk. ]. Biol. Chem. 266, 10050-10053 (1991).
Biological Trace Element Research
Vol...33, 1992
