Heterogeneity of insulin-like growth factor binding proteins in swine

Proteins in porcine amniotic fluid and sera (both fetal and adult) were separated electrophoretically in sodium dodecyl sulfate-containing polyacrylamide gels and transferred to nitrocellulose sheets. Western blots were analysed for proteins that would bind (a) radioiodinated insulin-like growth factor-I (IGF-I) and (b) antibodies to a rat insulin-like growth factor binding protein. Multiple insulin-like growth factor binding proteins were identified in sera. The binding proteins ranged in size from Mr 192,000 to 26,000. One immunologically cross-reactive protein (Mr 36,000) was detected. No binding proteins were detected routinely in amniotic fluids. Sera from adult swine were fractionated by preparative isoelectric focusing. Two binding proteins (Mr 192,000, 46,000) were located in acidic fractions which also contained IGF-I and IGF-II. Two binding proteins (Mr 36,000, 26,000) were located in neutral to basic fractions which contained primarily IGF-II. Immunoglobulin-sized material from adult sera fractionated over Sephadex G-200 contained two binding proteins (Mr 46,000, 42,000) whereas albumin-sized material contained one (Mr 36,000). Porcine insulin-like growth factor binding proteins are as heterogeneous as those from humans.