Human Leukocyte Migration Inhibitory Factor (LIF)

Abstract

Previously reported experiments suggested that an esterase or a protease, or both, might participate in the expression of human LIP. To clarify this further a wide variety of simple esters were tested for the ability to protect LIF against inactivation by the serine esterase inhibitor phenylmethylsulfonyl fluoride (PMSF). .alpha.-N-benzoyl-L-arginine ethylester (BAEE), a typical trypsin substrate, and bis-p-nitrophenyl phosphate (BNPP), a phosphodiester, were the only esters capable of retaining LIF activity in the presence of PMSF. Agents chemically closely related to these esters were inactive. The protection afforded by BAEE and BNPP was the kind that would be anticipated if the esters and irreversible inhibitor competed for the same site on LIF. BAEE and BNPP also protected against inactivation by di-isopropyl-fluorophosphate (DFP), another irreversible serine esterase inhibitor. LIF-treated leukocytes partly escaped migration inhibition in the presence of BAEE and BNPP, respectively. Human LIF apparently contains a serine residue necessary for lymphokine activity. It is not proved that LIF as an enzyme is capable of hydrolyzing BAEE and BNPP, although it seems highly possible. The substrate specificities of a putative LIF enzyme are discussed on the basis of the chemical structure of BAEE and BNPP.