The serum proteins of late-fetal and neonatal unsuckled piglets have been compared with those of 5-day-old and older pigs. Electrophoretic behavior on polyacrylamide gel (PAG) and starch block, immunoreactivity, and solubility in 5% trichloroacetic acid (TCA) were studied. Neonatal sera are nearly devoid of albumin electrophoretically, and precipitin analysis indicates an albumin concentration of 2 mg/ml, compared with 35 mg/ml in maternal sow serum. Albumin rises postnatally to 15 mg/ml by 4-5 days of age and 29 mg/ml by 10 days. Most of the protein of neonatal serum occurs in two prominent electrophoretic bands, moving faster and slower than albumin. The proteins occurring in these bands retain their distinct migration rates on mixing with each other and with albumin, and they can be separated from each other electrophoretically on starch block. Despite this, these prominent neonatal serum proteins exhibit extensive immunologic cross-reactivity with each other. Most (60-85%) of the proteins of neonatal serum are soluble in cold 5% TCA. The TCA-soluble proteins migrate mainly as prealbumin and postalbumin on 7 1 2% PAG electrophoresis, but react with antialbumin. Treatment of neonatal pig serum by a procedure which isolates bovine fetuin (Spiro, 1960) also yields pre- and postalbumins, but this preparation does not react with antialbumin. The major electrophoretic difference between the latter preparation and the TCA-soluble proteins is a division, seen best on 10% PAG, of the prealbumin band of TCA-soluble proteins into two bands, one of which is sharp and migrates at the leading edge of the albumin zone. The presence of such a band in serum fractions from newborn animals correlates with the reactivity of such fractions with antialbumin. It thus appears that the albumin in newborn (and fetal) sera is unusually soluble in TCA and migrates somewhat faster than does albumin in serum from older animals. The typical electrophoretic moieties of neonatal serum and the TCA-solubility of its proteins disappear from serum nearly completely within 10 days after birth, but the same or related antigenic determinants can be demonstrated in adult serum. The postnatal changes in pig serum proteins are unusually abrupt but may be taken as broadly analogous to changes from fetal to adult serum protein patterns which occur in many mammals. The piglet is thus a useful model system in which to study these changes. © 1969.
