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ExtractSerum concentrations of hemopexin, transferrin, and albumin were measured for 12 fetuses between 14 and 36 weeks of gestational age. Hemopexin levels ranged from 7 to 64 pg/ml, transferrin levels ranged from 280 to 928 pg/ml, and albumin levels ranged from 13 to 59 mg/ml. In general, the serum concentrations of these three proteins increased with advancing gestation.Placenta, thymus, and colon did not incorporate 14C-labeled amino acids into hemopexin, transferrin, or albumin. By contrast, radioimmune precipitates for five culture supernatants of liver indicated significant synthesis of albumin and hemopexin.[MC]Albumin accounted for 22-73% and [14C]hemopexin 1.1-4.2% of the total 14C-labeled proteins. In each instance, the ["Cltransferrin was below 1% of the total l 4 C-labeled proteins. SpeculationThese data show considerable synthesis of albumin and hemopexin, but not transferrin, by the liver during fetal life. That hemopexin levels are lower in fetal than adult sera may reflect an increased requirement for heme disposal. Possibly, the increased turnover of hemopexin is related to the shortened lifespan of fetal in comparison with adult erythrocytes.Mammalian serum contains two major porphyrin-binding proteins, hemopexin and albumin (1 I). Hemopexin has a higher binding affinity for metalloporphyrins than albumin (15) and is instrumental in the disposal of heme (5,8,9,14,19). The liver is the site of hemopexin synthesis in rhesus monkeys (17), rabbits (17), and rats (12). In the present study, human fetal tissues were cultured and examined for the relative amounts of hemopexin, albumin, and transferrin produced. MATERIALS A N D METHODS FETAL TISSUESSerum samples of fetuses were obtained by heart puncture.Tissuc culturcs of livcr, placenta, thymus, a n d colon of h u m a n fetuses, 12-25 weeks old, were developed by Dr. Peter F. Kohler (20), employing [14C]lysine and [14C]isoleucine (4). Culture supernatants were dialyzed against 0.015 M phosphate-buffered saline (PBS), pH 7.4, lyophilized, dissolved in distilled water (0.2-0.3 ml), shipped to La Jolla on Dry Ice, and stored at 7 0°.Tissues for absorption of antisera were obtained from fetuses within 5 hr of saline-induced abortion. Small pieces of liver, placenta, thymus, and colon were passed through a stainless steel mesh with the aid of screen rakes. Cells were washed three times with PBS and mixed with 3 volumes antiserum/l volume of cells. The mixtures were incubated at 37O for 30 min, at 4' for 48 hr, and then the cells were removed by centrifugation at 1,200 x g for 30 min. T o 5-50-pl aliquots of culture supernatant, 50 1 1 normal human serum as carrier protein were added and the nonspecific radioactivity was precipitated by addition of equivalent amounts of R G G and anti-BGG. T h e mixture was incubated 37' for 30 min, at 4' overnight, and centrifuged at 1,200 x g for 30 min at 4'. The supernatants were then used for precipitation with the monospecific antisera added to antibody excess. The reaction mixtures were incubated and centrifuged as...