Human Leucocyte Migration Inhibitory Factor (LIF)

The cyclic nucleotides guanosine 3',5'-cyclic monophosphoric acid (3',5'-cGMP) and cytidine 2',3'-cyclic monophosphoric acid (2',3'-cCMP) but not cyclic phosphodiesters derived from the bases adenine and uracil preserved LIF activity against the blocking effect of the serine protease inhibitor phenylmethylsulphonyl fluoride (PMSF). Phosphomonoesters derived from guanosine and cytidine as well as 2',3'-cGMP and 3',5'-cCMP were all inactive, indicating specificity for phosphodiester bonds and their respective positions in the two active nucleotides. The protection afforded by 3',5'-cGMP and 2',3'-cCMP was dose dependent. Thus, using 10(-3) M PMSF, 3',5'-cGMP was active at concentrations higher than 10(-5) to 10(-4) M, and 2',3'-cCMP at concentrations higher than 3 X 10(-4) to 10(-3) M. The more pronounced LIF-inhibitory effect obtained by increased concentrations of PMSF could be overcome by raising the levels of the nucleotides, indicating that the interactions between PMSF and the nucleotides with LIF were mutally exclusive. The possibility that 3',5'-cGMP and perhaps 2',3'-cCMP function as modulators of LIF is discussed, and models for the function of this lymphokine are proposed.