Labeling of phospholipids in vesicles and human erythrocytes by photoactivable fatty acid derivatives

Engineered cell surfaces: fertile ground for molecular landscaping

The cell surface contains a wealth of information that determines how cells interact with their environment. Methods for directing the cell surface expression of novel protein-based and oligosaccharide-based epitopes are stimulating new directions in biotechnology and biomedical research.

Photochemical labeling of human erythrocyte membranes with radioiodinatable azidosalicylic acid derivative of globoside

In an attempt to define glycolipid functions we have prepared photoactivatable, iodinatable derivative of globoside and used it for photoaffinity labeling of human erythrocyte membranes. Lysogloboside (Gb4Sph) was prepared from globoside through deacylation in methanolic KOH followed by re-N-acetylation of galactosaminyl residue. The NH2 group of sphingosine residue in Gb4 Sph reacted with N-hydroxysuccinimidyl-4-azidosalicylic acid resulting in the formation of Gb4Sph-ASA which was purified by preparative tlc and column chromatography. It migrated on tlc as a single spot in two solvent systems, was susceptible to leech ceramide glycanase and could be radioiodinated to a specific radioactivity of about 200 Ci/mmol. Gb4Sph-[125I]ASA was incorporated into human erythrocytes in a time and concentration-dependent manner. Before photolysis 96% of the Gb4Sph-ASA could be removed with albumin but not with trypsin. After photolysis about 50% of the label was firmly bound to erythrocytes being resistant to albumin and trypsin treatment. The label was distributed between membrane proteins and lipids in about 1:2.3 ratio. Photolabeled proteins were analyzed by SDS-PAGE followed by autoradiography and immunostaining. Most of the radioactivity was detected in band 3 and its proteolytic fragments irrespective of the duration of photolysis. Photolabeling of erythrocyte lipids was demonstrated by Sephadex LH-20 column chromatography.

Transbilayer migration (flip-flop) of 12-(4-azido-2-nitrophenoxy)stearoyl glucosamine in large unilamellar phospholipid vesicles

The ability of the glycolipid photoprobe, 12-(4-azido-2-nitrophenoxy)-stearoyl[1-14C]glucosamine (12-APS-GlcN), to undergo transbilayer flip-flop and intermembrane transfer between liposomes was examined. It was found that probe which was incorporated into membranes during the preparation of large unilamellar vesicles (LUVs) could be rapidly and completely extracted by incubation of these donor vesicles (in the liquid-crystalline state) with probe-free acceptor vesicles.

Phospholipids involved in specific binding of 12-O-(5-azido-2-nitrobenzoyl)phorbol-13-acetate to epidermal microsomes, a photolabeling study

The preparation of 12-O-(5-azido-2-nitro)benzoylphorbol-13-acetate (NABPA) is described. It is used as a photoaffinity probe to study the biochemical components involved in the specific binding of phorbol esters to an epidermal particulate fraction (microsomes) from NMRI mice: without irradiation NABPA binds in a saturable and high affinity manner (KD = 12 nM; Rt = 2.6 pmol/mg protein) to microsomes; after irradiation (at 350 nm) specific photolabeling (representing specific binding of NABPA) is found of phospholipids (phosphatidyl-serine (PS) and -ethanolamine(PE)), but not of protein. The results are discussed in the context of protein kinase C being a receptor for phorbol esters.

Identification of membrane proteins of human blood platelets with a hydrophobic photolabel

A photoactivable glycolipid probe, 12-(4-azido-2-nitrophenoxy)stearoyl[1-14C]glucosamine, was used to label proteins and lipids of platelet membranes. The proteins were analyzed by two-dimensional high-resolution gelelectrophoresis. The labeling patterns showed that three membrane proteins were labeled which were not previously identified by ectolabeling (Sixma, J.J. and Schiphorst, M.E. (1980) Biochim. Biophys. Acta 603, 70-83). Analysis of the lipid fraction showed that phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine were labeled by the probe. The distinct labeling of phosphatidylserine strongly suggests that the probe redistributes between the two halves of the bilayer.