Phosphorylation of Minor Lipids of Human Milk Tightly Bound to Secretory Immunoglobulin A.

Catalytic antibodies or abzymes possessing the different catalytic activities were detected in the sera of patients with various autoimmune pathologies, where their presence is most probably associated with autoimmunization. Normal humans are generally considered to have no abzymes, since no obvious immunizing factors are present. The ability of small fraction of sIgA from human milk to phosphorylate selectively casein in the presence of gamma(32)P-ATP was previously shown to be a property of the Abs. Here we revealed for the first time that the same small fraction of sIgA contains Ab subfraction, which is tightly bound to unusual minor lipids of human milk. The lipids may be phosphorylated in the presence of gamma(32)P-ATP and remain partially bound to Abs after purification of polyclonal sIgA by several sequential chromatographies. These lipids may be effectively separated from sIgA only by gel filtration in a buffer containing 5% dioxane or by extraction of the sIgA solutions with chloroform-methanol mixture. It has been shown that two minor milk phospholipids, similarly to the previously described GM1, GM3 and GD3 gangliosides contain residue of sialic acid, but in conrast to the latter lipids, they can not be oxidized with sodium periodate; alkaline methanolysis of them results in formation of 4 and 5 products respectively, while hydrolysis of the gangliosides gives only one or two products. All the data obtained give an evidence in favor of that the minor lipids tightly bound to sIgA may be considered as new lipids of human milk. It was suggested that phosphorylation of the lipids is catalyzed by sIgA antibodies.