Lipid kinase activity of antibodies from milk of clinically healthy human mothers.

We have shown recently that polyclonal human milk sIgA contains a subfraction of antibodies (Abs) tightly bound to unusual minor milk lipids containing sialic acid. Here, we show that a small subfraction of milk IgG is tightly bound to the similar or the same minor lipids. The ability of small fractions of sIgA and IgG from human milk to phosphorylate selectively two minor lipids in the presence of [gamma-(32)P]nucleoside triphosphates was shown here for the first time to be an intrinsic property of these antibodies. In contrast to known kinases, antibodies with lipid kinase activity can transfer phosphoryl group to lipids not only from ATP but also from other different nucleotides (dATP, GTP, dGTP, UTP, TTP) with comparable efficiencies (30-100%). To our knowledge, there are no examples of enzymes using orthophosphate as a substrate of phosphorylation reactions. An extremely unusual property of lipid kinase Abs is their high affinity for orthophosphate (K(m)=1.6-5.6 microM) and capability to phosphorylate minor lipids using [(32)P]orthophosphate as donor of phosphate group. The relative specific activity and affinity of abzymes for orthophosphate and ATP depend significantly on donor milk. However, the levels of Ab-dependent phosphorylation of lipids for all Abs in the case of ATP (100%) and orthophosphate (60-80%) as substrates are comparable. The first example of natural abzymes with synthetic activity was milk sIgA with protein kinase activity. Most probably, lipid kinase sIgA and IgG of human milk are the second example of Abs with synthetic activity.