Demonstration of interleukin 1 activity in apparently homogeneous specimens of the pI 5 form of rabbit endogenous pyrogen

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Abstract
Rabbit mononuclear cells from oil-induced peritoneal exudates were purified by centrifugation on Percoll gradients, suspended in tissue culture medium, and stimulated with opsonized Staphylococcus epidermidis. The supernatants from these macrophages caused fever when injected intravenously into rabbits (endogenous pyrogen [EP] activity). The EP activity was contained in two protein fractions, with pIs of 7.3 and ca. 5.0. The same fractions caused mouse thymocytes to incorporate tritiated thymidine when incubated in vitro with small quantities of phytohemagglutinin (interleukin 1 [IL-1] activity). The pI 5.0 form of EP was purified to apparent homogeneity by sequential use of ammonium sulfate precipitation, gel filtration, ion-exchange chromatography, hydrophobic chromatography, and high-resolution isoelectric focusing. EP and IL-1 activities were not separable by any of these procedures. Active fractions from isoelectric focusing were analyzed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Only one band was visible as judged by a silver staining method, and IL-1 activity could be recovered by renaturing eluates from the same region of sodium dodecyl sulfate gels run in parallel. An estimate of specific activity was made by comparing the intensity of stained bands of EP with the intensity of bands containing known quantities of lysozyme or RNase. By this criterion, the specific activity of purified pI 5 EP was between 17,000 and 58,000 degrees C U/mg of protein, and the specific activity in terms of IL-1 was between 59 million and 360 million U per mg of protein. These observations suggest that both EP and IL-1 activities can be expressed by a single molecular species. The implications of this coincidence are discussed. It was also shown that highly purified pI 5 EP obtained from macrophages stimulated in the presence of 14C-labeled amino acids contained significant 14C radioactivity. This suggests that the pI 5.0 EP, like the pI 7.3 form, was synthesized de novo from amino acid precursors.