Immunological comparison of phosphatidylinostiol and phosphatidylcholine exchange proteins in bovine brain, liver and heart.

The two phosphatidylinositol exchange proteins isolated from bovine cerebral cortex, I (isoelectric point pH 5.2) and II (isoelectric point pH 5.5), had essentially identical amino acid compositions. Rabbit antisera preparations specific to each of these brain proteins were equally effective in inhibiting the phosphatidylinositol transfer activity of both protein I and II. Judged by double diffusion on agar gels, immunoprecipitation was not observed between either of the brain phosphatidylinositol exchange proteins and anti-liver phosphatidylcholine exchange protein antibody or between liver phosphatidylcholine exchange protein and anti-brain phosphatidylinositol exchange protein antibody. Phosphatidylinositol and phosphatidylcholine transfer activity was measured in microsome-liposome assay systems. For membrane-free tissue preparations phosphatidylinositol activity increased in the order: brain greater than heart greater than liver, while phosphatidylinositol exchange proteins transferred phosphatidylinositol and phosphatidylcholine in the ratio 1.4: liver phosphatidylcholine exchange protein transferred exclusively phosphatidylcholine. Phosphatidylinositol transfer activity in brain, heart and liver was more than 80% inhibited by anti-brain phosphatidylinositol exchange protein antibody. The proportion of phosphatidylcholine transfer activity sensitive to anti-liver phosphatidylcholine exchange protein antibody was 15% for brain, 75% for liver and 20% for heart, while the proportion sensitive to anti-brain phosphatidylinositol exchange protein antibody was 65% for brain, 10% for liver and 60% for heart. Together these two classes of phospholipid exchange proteins accounted for approx. 80% of the phosphoatidylcholine transfer activity in selected bovine tissues. A protein which was chemically, immunologically, and catalytically similar to liver phosphatidylcholine exchange protein was identified in brain and contributed about 20% of the phosphatidylcholine transfer activity in that tissue.