Calcium inhibits diacylglycerol uptake by serum albumin

Serum albumin is an abundant protein in blood plasma, that is well-known for its ability to transport hydrophobic biomolecules and drugs. Recent hypotheses propose that serum albumin plays a role in the regulation of lipid metabolism in addition to its lipid transport properties. The present work explores the capacity of bovine serum albumin (BSA) to extract diacylglycerols (DAG) from phospholipid bilayers, and the inhibition of such interaction by divalent cations. Quantitative measurements using radioactive DAG and morphological evidence derived from giant unilamellar vesicles examined by confocal microscopy provide concurrent results. BSA extracts DAG from vesicles consisting of phosphatidylinositol/DAG. Long, saturated DAG species are incorporated more readily than the shorter-chain or unsaturated ones. Divalent cations hinder DAG uptake by BSA. For Ca(2+), the concentration causing half-maximal inhibition is approximately 10 muM; 90% inhibition is caused by 100 muM Ca(2+). Sr(2+) requires concentrations one order of magnitude higher, while Mg(2+) has virtually no effect. As an example on how DAG uptake by BSA, and its inhibition by Ca(2+), could play a regulating role in lipid metabolism, a PI-specific phospholipase C has been assayed in the presence of BSA and/or Ca(2+). BSA activates the enzyme by removing the end-product DAG, but the activation is reverted by Ca(2+) that inhibits DAG uptake.