Analysis of membranes photolabeled with lipid analogues. Reaction of phospholipids containing a disulfide group and a nitrene or carbene precursor with lipids and with gramicidin A

Head group and structure specific interactions of enkephalins and dynorphin with liposomes: investigation by hydrophobic photolabeling

The interaction of the opioid enkephalins and endorphins with lipid bilayers is largely unknown. Such interactions might, however, be important for understanding the molecular mechanisms of biological action. We have therefore studied the interaction of several enkephalins and of dynorphin-(1-13)-tridecapeptide (dynorphin1-13) with model membrane systems, using the extremely hydrophobic photolabel of J. Brunner and G. Semenza, (Biochemistry 20, 7174-7182 (1981)) 3-trifluoromethyl-3-(m[125I]iodophenyl)diazirine. By observing several limitations of the method, it was possible to characterize hydrophobic interactions of opioid peptides with liposomes prepared from egg yolk phosphatidylcholine (PC) plus dipalmitoylphosphatidic acid (PA), from brain phosphatidylserine (PS) alone, and from brain cerebroside sulfate (CS) alone, Dynorphine1-13 exhibited strong hydrophobic interactions through its N-terminal "message" segment which were potentiated by the "address" that itself remained in the aqueous phase. This behavior was consistent with the reported pharmacological potentiation. The enkephalins were generally weakly labeled in the PC/PA and PS systems. However, in the CS systems the preferentially mu agonists were labeled very strongly whereas the preferentially delta agonists were labeled more weakly yet. The kappa agonist, dynorphin1-13, was strongly, but more equally labeled in the three systems. Thus, there was a head group specificity that, in our series of compounds, correlated with opiate receptor sub-type specificity. The results were consistent with the behavior of the mu agonist, morphine.

[125I]Iodonaphtylazide labeling selectively a cysteine residue in the F0 of the ATP-synthase from E. coli is unsuitable for topographic studies of membrane proteins

The ATP synthase from E. coli was reacted with the hydrophobic photolabel [125I]iodonaphtylazide. Subunit b in the F0-part was selectively labelled. Label was traced back to the single cysteine21 in subunit b. Thus the reactive intermediate of INA generated by photolysis had a high preference for nucleophiles. Due to this high selectivity the detection of membrane spanning peptide segments by labelling with INA is not reliable.

Phorbol ester receptors and the in vitro effects of tumor promoters

The evidence for the multistage nature of tumor promotion in vivo and for multiple subclasses of phorbol ester receptors in vitro argues that multiple mechanisms of tumor promotion exist. The existence of multiple mechanisms suggests that brute force assay for tumor promoters in vivo may be inadequate and that understanding of mechanisms may be essential. The interest in the phorbol esters is not primarily that they are environmental hazards for man, but rather that they provide a probe for phorbol ester receptors. These receptors are found in people, and modulation of their activity may play a role in tumor promotion in man.

Identification of the lipid-binding site of phosphatidylcholine-transfer protein with phosphatidylcholine analogs containing photoactivable carbene precursors

The lipid binding site of the phosphatidylcholine transfer protein from bovine liver has been investigated by use of phosphatidylcholine analogs which carry a diazirinophenoxy group linked to the omega-carbon of either the sn-2-[1-14C]hexanoyl (PC I) or sn-2-[1-14C]undecanoyl chain (PC II). Photolysis of the PC I(PC II)-transfer protein complex resulted in a covalent coupling of 30-40% of the label to the protein as shown by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Upon mild alkaline treatment of the photolysed complex the protein containing covalently coupled 14C-label was separated from the noncoupled 14C-label by gel permeation chromatography. The 14C-labeled protein was degraded with protease from Staphylococcus aureus, trypsin and cyanogen bromide and specific 14C-labeled peptides were sequenced by automated Edman degradation. Major sites of coupling shown by release of radioactivity were identified as Tyr54 and the peptide segment Val171-Phe-Met-Tyr-Tyr-Phe-Asp177. Both PC I and PC II coupled extensively to Tyr54 (90% and 50% of total labeling, respectively). The remainder of the radioactivity was released from the peptide Val171-Asp177 with a distinct difference in in the pattern of release depending on whether PC I or PC II were used. Thus, coupling occurred preferentially to Tyr175 and Asp177 with PC I while Val171 and Met173 were labeled preferentially with PC II. This shift in coupling is compatible with an increase of 0.6 nm for the sn-2-fatty-acyl chains of PC I and II, assuming that the peptide Val171-Asp177 has adopted the strongly predicted beta-strand configuration. These data have been interpreted in terms of the localization of phosphatidylcholine in the phosphatidylcholine transfer protein.

Membrane topology of ATP synthase from bovine heart mitochondria and Escherichia coli

The polypeptides exposed to lipids in the membranous F0 sector of the mitochondrial and Escherichia coli ATP synthases were labelled with radioactive photoreactive lipids. Highly resolving gel electrophoretic conditions were used in order to separate all the eighteen components forming the bovine heart mitochondrial enzyme. The hydrophobic labelling was performed on fully active and inhibitor-sensitive ATP synthases. In the mitochondrial enzyme prepared according to Serrano et al. (1976) [J. Biol. Chem. 251, 2453-2461] seven polypeptides of Mr 30500; 11500; 10500; 10000; 9500; 8500 and 4500 were labelled. The major amount of radioactivity was associated with the 30500-Mr component, which is thought to be the adenine nucleotide carrier. In the preparation of Galante et al., (1979) which almost completely lacks this component [J. Biol. Chem. 254, 12372-12378] nine polypeptides of Mr 25000; 21000; 11500; 10500; 10000; 9500; 9200; 8500 and 4500 were labelled. In the ATPase synthase from E. coli the major amount of labelling was associated with subunit b and only a minor portion with subunit c.

A carbene-generating photoaffinity probe for beta-adrenergic receptors

A new radioiodinated (2.2 Ci/mumol) iodocyanopindolol derivative carrying a 4-(3-trifluoromethyldiazirino)benzoyl residue has been synthesized. The long-wavelength absorption of the diazirine permits formation of the carbene by photolysis under very mild conditions. [125I]ICYP-diazirine binds with high affinity (Kd = 60 pM) to beta-receptors from turkey erythrocyte membranes. Upon irradiation, [125I]ICYP-diazirine is covalently incorporated in a Mr 40 000 protein. Stereoselective inhibition of photolabeling by the (-)enantiomers of alprenolol and isoproterenol indicated that the Mr 40 000 protein contains a beta-adrenergic binding site. The yield of specific labeling was up to 8.2% of total beta-receptor binding sites. The Mr 40 000 protein photolabeled in the membrane could be solubilized at comparable yield with either digitonin or Triton X-100. Irradiation of digitonin-solubilized turkey erythrocyte membranes with [125I]ICYP-diazirine resulted in specific labeling of two proteins with Mr 40 000 and 50 000. In guinea-pig lung membranes, at least five proteins were photolabeled, of which one (with approximately Mr 67 000) was labeled specifically.

Selective labeling of the hydrophobic core of membranes with 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine, a carbene-generating reagent

The synthesis of a new photoactivatable probe, 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine ([125I]-TID), with a high specific radioactivity (10 Ci mmol-1) is described. It was tested as a probe for the hydrophobic core of membranes. TID partitions strongly in favor of the lipid phase of membranes, and the photogenerated carbene labels intrinsic membrane proteins in a highly selective manner. This conclusion was reached from the distribution of radioactivity among the proteins of [125I]TID-labeled human erythrocyte membranes. By far the most heavily labeled protein is band 3 [nomenclature of Fairbanks, G., Steck, T. L., & Wallach, F. G. H. (1971) Biochemistry 10, 2606-2617] while the labeling of glycophorin is approximately 5 times less than that of band 3. There is little or no labeling of known extrinsic proteins.

Preparation of photoreactive derivatives of glutathione and [9-(2-mercaptotryptophan)]corticotropin by selective modification of the sulfhydryl group

The photoreactive arylsulfenyl chlorides 2-nitro-4-azidophenylsulfenyl chloride (2,4-NAPS-Cl) and 2-nitro-5-azidophenylsulfenyl chloride (2,5-NAPS-Cl) have been used for the selective modification of thiol groups in glutathione and [Trp(SH)9]corticotropin (ACTH). Both reagents reacted rapidly with both types of thiol groups to form unsymmetrical disulfides. The photoreactive derivatives of glutathione and [Trp(SH)9]ACTH were stable to neutral and acidic conditions but were readily cleaved above pH 9 and by beta-mercaptoethanol. Photolysis of the NAPS derivatives of [Trp(SH)9]-ACTH at neutral pH resulted in the formation of covalently liked polymers and dimers which yielded monomer upon treatment with beta-mercaptoethanol. Analysis of the amino acid composition of acid hydrolysates of photolysed monomeric and dimeric products indicated a decrease in proline, valine, tyrosine, and phenylalanine.