Human leukocyte inhibitory factor (LIF): two distinct molecular species

Human leukocyte inhibitory factor or LIF was generated in vitro by stimulating blood lymphocytes with concanavalin A (Con A). The control and Con A active supernatants were partially purified by gel filtration on Sephadex G-100. The fraction containing LIF (68,000 daltons) activity was then subjected to isoelectric focusing (pH 3 to 10 ampholines) in a sucrose gradient. Two LIF activities were reproducibly recovered by this procedure. One molecular form was found to have an isoelectric point of approximately pH 5.0 and the other approximately pH 8.5. Both molecular species were rechromatographed on Sephadex G-75 and found to have the same apparent m.w. (68 to 75,000). Furthermore, the biologic activity of both factors was destroyed after treatment with diisopropylphosphofluoridate, suggesting that they may be esterases.