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G M Helmkamp

Publications and source records attributed to G M Helmkamp.

52 records · Page 3Linked to original sources
PubMed1980

Interaction of bovine brain phospholipid exchange protein with liposomes of different lipid composition.

The major phospholipid exchange protein from bovine brain catalyzes the transfer of phosphatidylinositol and phosphatidylcholine between rat liver microsomes and sonicated liposomes. The effect of liposomal lipid composition on the transfer of these phospholipids has been investigated. Standard liposomes contained phosphatidylcholine-phosphatidic acid (98 : 2, mol%); in general, phosphatidylcholine was substituted by various positively charged, negatively charged, or zwitterionic lipids. The transfer of phosphatidylinositol was essentially unaffected by the incorporation into liposomes of phosphatidic acid, phosphatidylserine, or phosphatidylglycerol (5--20 mol%) but strongly depressed by the incorporation of stearylamine (10--40 mol%). Marked stimulation (2--4-fold) of transfer activity was observed into liposomes containing phosphatidylethanolamine (2--40 mol%). The inclusion of sphingomyelin in the acceptor liposomes gave mixed results: stimulation at low levels (2--10 mol%) and inhibition at higher levels (up to 40 mol%). Cholesterol slightly diminished transfer activity at a liposome cholesterol/phospholipid molar ratio of 0.81. Similar effects were noted for the transfer to phospholipidcholine from microsomes to these various liposomes. Compared to standard liposomes, the magnitude of Km tended to increase for liposomes which depressed phospholipid transfer and to decrease for those which stimulated; little change was observed in the values of V. Single phospholipid liposomes of phosphatidylinositol were inhibitory when added to standard liposomes. Because bovine brain phospholipid exchange protein is able to distinguish among a wide spectrum of membrane interfaces, taking into account variations in the polar head groups as well as the fatty acyl moieties of the liposomal phospholipids, it may be considered a reasonable model system for protein-lipid and protein-membrane interactions.

Animals↗
PubMed1976

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.

Amino Acids↗
PubMed1975

Kinetic model of the protein-mediated phosphatidylcholine exchange between single bilayer liposomes.

The phosphatidylcholine exchange protein from beef liver catalyzes the exchange of phosphatidylcholine between single bilayer liposomes (Hellings et al. (1974), Eur. J. Biochem. 47, 601). A model has been proposed which describes the kinetics of this exchange. Steady-state equations have been derived from the model and have been used for the derivation of the theoretical rate equation. Computer analysis shows a good fit with the experimental results. It follows from the analysis that the apparent dissociation constant of the exchange protein-liposome complex decreases with an increasing phosphatidic acid content of the liposomes. This suggests that in this model system it is the phospholipid composition of the membranes involved that regulates the amount of exchange protein available to function as a carrier of phosphatidylcholine.

Animals↗