Functional differentiation of amphiphilic helices of the apolipoproteins by hydrophobic moment analysis

Discovery of Novel Caeridins from the Skin Secretion of the Australian White's Tree Frog, Litoria caerulea

Abundant biologically active peptides have been discovered from frog skin secretions, a rich natural source of bioactive compounds with great potential in drug discovery. In this study, three Caeridin peptides, namely, Caeridin-1, S5-Caeridin-1, and Caeridin-a1, were discovered from the skin secretion of the Australian White's tree frog, Litoria caerulea, for the first time, by means of combining transcriptomic and peptidomic analyses. It also represents the first report on bioactive Caeridins since this family of peptides was initially studied 20 years ago. Chemically synthetic versions of each natural Caeridin demonstrated promising bioactivities either on rat smooth muscles or against microbial growth. Specifically, Caeridin-1 produced contraction of rat bladder smooth muscle, while S5-Caeridin-1 induced relaxation of rat ileum smooth muscle, both at nanomolar concentrations. Moreover, Caeridin-a1 was shown to potently inhibit the growth of the planktonic Gram-positive bacteria Staphylococcus aureus (S. aureus), methicillin-resistant S. aureus (MRSA), and Enterococcus faecalis (E. faecalis), the Gram-negative bacterium, Escherichia coli (E. coli), and the yeast, Candida albicans (C. albicans). The discovery of these Caeridins may induce further intensive and systematic studies of frog skin peptides to promote the discovery of natural templates as lead compounds for drug discovery and therapeutic application.

Expression and conservation of apolipoprotein AIV in an avian species

In birds, intestinally derived lipoproteins are thought to be secreted directly into the portal vein rather than to enter the circulation via the lymphatic system as in mammals. Hepatic clearance of these so-called portomicrons must be rapid, but the protein(s) mediating their catabolism, presumably analogues of the 36-kDa mammalian apolipoprotein E, have not been identified. In searching for such a mediator(s), we have isolated a hitherto unknown 38-kDa protein from chicken serum, which we identified by microsequencing and molecular cloning as a counterpart to mammalian apolipoprotein AIV (apoAIV). Mature chicken apoAIV consists of 347 amino acids, lacks cysteine residues, and displays 57% sequence identity with human apoAIV and, to a significantly lesser extent, with apoAIVs of rodents. This first nonmammalian apoAIV characterized is the smallest homologue reported so far, because of the lack of repeated motifs at the carboxyl terminus with the consensus sequence Glu-Gln-Glu/Ala-Gln, a hallmark of mammalian apoAIVs. Chicken apoAIV (isoelectric point, 4.65) is also considerably more acidic than its human counterpart. Agarose gel electrophoresis revealed that unlike human apoAIV, which migrates to a pre-alpha-position, chicken apoAIV shows fast alpha migration. Functional characterization demonstrated that the avian protein is able to activate the enzyme lecithin:cholesterol acyltransferase. Roosters and hens express apoAIV predominantly in the gut, one-fifth as much in the liver, and no other sites of expression are identifiable by Northern blot analysis. Although pronounced intestinal synthesis is common to apoAIVs, the features of the avian protein support the notion that it represents a prototype of an apoprotein that evolved to acquire possibly distinct functions in mammals and birds.

Design of a new class of amphipathic helical peptides for the plasma apolipoproteins that promote cellular cholesterol efflux but do not activate LCAT

Amphipathic helical peptides represent the lipid-binding units of the soluble plasma apolipoproteins. Several synthetic peptide analogues have been designed to mimic such structures and have been used to unravel some of the mechanisms involved in the physiological function of the apolipoproteins, including lipid binding, LCAT activation, and enhancement of cholesterol efflux from lipid-laden cells. A series of novel synthetic peptides, named ID peptides, was modeled on the basis of the structural properties common to the amphipathic helices of apolipoprotein (apo) A-I. In these new peptides, however, the segregation between hydrophobic and hydrophilic faces of the helices is more pronounced than in apoA-I, so that the surface of the hydrophobic and hydrophilic faces of the amphipathic helices is equal. Moreover, there are fewer negatively charged residues in the center of the hydrophilic face of the helical peptides. Most charged amino acids are located along the edge of the helix and are susceptible to forming salt bridges with residues of an antiparallel helix, such as around a discoidal phospholipid/peptide complex. The physicochemical characteristics of these peptides and their complexes with phospholipids were compared with those of the 18A peptide and its lipid/peptide complex. All ID peptides bind dimyristoylphosphatidylcholine vesicles more rapidly than the 18A peptide to yield discoidal peptide/phospholipid complexes of comparable size. The alpha-helical content of the lipid-free ID peptides is close to that of the 18A peptide and increases slightly on lipid binding. The stability of the ID and 18A peptides and of the phospholipid/peptide complexes against guanidinium hydrochloride denaturation is higher than that of lipid-free and lipid-bound apoA-I. LCAT activation by the 18A/phospholipid/cholesterol complexes equals that of apoA-I/ phospholipid/cholesterol complexes, whereas none of the ID peptides tested is able to activate LCAT to a significant extent. Incubation of the peptide/phospholipid complexes with lipid-laden macrophages induces cellular cholesterol efflux and incorporation of cholesterol into the complexes. The cholesterol efflux capacity of the peptide/phospholipid complexes is comparable among the peptides and higher than that of apoprotein/phospholipid complexes. In conclusion, although the amphipathicity of the new peptides is higher than that of the 18A model peptide, the lack of LCAT activation by the ID peptides suggests that an enhanced segregation of the hydrophobic and hydrophilic residues, equal magnitude of hydrophobic and hydrophilic faces of the helix, and the absence of negatively charged residues in the central part of the hydrophilic face might account for the lack of LCAT activity of these peptides. These parameters do not affect the capacity of the peptide/phospholipid complexes to promote cellular cholesterol efflux.

Branched synthetic constructs that mimic the physico-chemical properties of apolipoprotein AI in reconstituted high-density lipoproteins

Amphipathic helical repeats are considered as the structural units of numerous apolipoproteins and have been described as being responsible for the interaction of apolipoproteins with phospholipids in high-density lipoproteins (HDL). Furthermore, apolipoproteins, and especially apolipoprotein AI (apoAI), are involved in various biological functions of these circulating particles in plasma. Studies with synthetic peptides corresponding to domains of the apoAI sequence have however shown that short 39-residue fragments do not interact strongly enough with phospholipids to generate particles that correctly mimic the physico-chemical properties of HDL reconstituted with native apoAI [Vanloo, B., Demoor, L., Boutillon, C., Lins, L., Baert, J., Fruchart, J. C., Tartar, A. & Rosseneu, M. (1995) Association of synthetic peptide fragments of human apolipoprotein A-I with phospholipids, J. Lipid Res. 36, 1686-1696.]. Here we show that synthetic branched multimeric peptides, often used as carriers for the design of synthetic vaccines (multiple-antigen peptides), can be used to mimic the physiochemical properties of apoAI in HDL. This type of molecule is obtained by using a small core matrix of Lys residues bearing radially branched synthetic peptides as dendritic arms. We compared the lipid-binding capacities and the structural properties of a linear peptide corresponding to residues 145-183 of apoAI [apoAI-(145-183)-peptide] with those of two multimeric peptides consisting respectively of three [trimeric apoAI-(145-183)] and four copies [tetrameric apoAI-(145-183)] of the selected sequence, branched on a covalent core matrix. This paper provides evidence for the increased abilities of the multimeric peptides to associate with phospholipids compared with the short linear peptides. Moreover, the trimeric apoAI-(145-183) peptide was most efficient in mimicking the physico-chemical and structural properties of native apoAI in reconstituted HDL. As tools adequate to unravel the structure/function relationship of separate apolipoprotein domains are still missing, these multimeric peptides might constitute an alternative approach to linear peptides which are poor mimetics and to protein mutants which are difficult to produce and only provide information about the total sequence.

Helix-helix interactions in reconstituted high-density lipoproteins

In this work we calculated the ionic interactions between adjacent amphipathic helices of apo A-I and apo A-IV. The calculation of the electrostatic potential around the helices helps identify the charged residues susceptible to form salt bridges between adjacent helices. An estimation of the stability of the different pairs of helices is derived from the calculation of the energy of interaction between contiguous helices at a water/lipid interface after energy minimization. The most stable energetic conformation corresponds to the 17-residue helices oriented anti-parallel and separated by a stretch of 5 residues in an extended beta-strand conformation, as calculated through the 'stereo alphabet' calculation procedure. In a pair of helices, the hydrophobic faces are directed towards the lipid core of the discoidal phospholipid-apolipoprotein complex and the hydrophobic lipid-protein interactions are major determinants for the stability of the complex. Interactions between polar residues located on the opposite face of the helix and water molecules can also contribute to the overall energy of the system. Finally, salt bridge formation between residues of opposite charge along the edge of the helical segments contribute to the cooperativity of the phospholipid-apolipoprotein complex formation. The mode of assembly of the amphipathic helical repeats of the apolipoproteins around the edge of a discoidal complex is therefore determined both by the hydrophobic character of the residues and by the charge complementarity along the edge of the helices which increases the structural stability and determines the relative orientation of the helices.(ABSTRACT TRUNCATED AT 250 WORDS)

Screening for naturally occurring apolipoprotein A-I variants: apo A-I(delta K107) is associated with low HDL-cholesterol levels in men but not in women

Isoelectric focussing (IEF) in carrier ampholyte-generated pH gradients and hybrid isoelectric focussing (HIEF) in immobilized pH gradients under nondenaturing conditions were used in parallel to screen 5,500 plasma samples for naturally occurring variants of apolipoprotein A-I (apo A-I). The following defects were identified in four unrelated subjects heterozygous for apo A-I variants: apo A-I(delta K107)(2 x), apo A-I(K107M)(1 x), and apo A-I(E41R)(1 x). The later variant is a novel finding. Family studies did not reveal any association of apo A-I(K107M) and apo A-I(E41R) with dyslipidemia, but identified several heterozygotes for apo A-I(delta K107) who had low levels of high density lipoprotein (HDL)-cholesterol. Therefore, and since the apo A-I(delta K107) is the most frequent apo A-I variant in Germany (1: 5,000) we evaluated our data and that reported from 11 families with 32 heterozygous carriers and 30 unaffected controls. This analysis revealed that apo A-I(delta K107) is associated with lower HDL-cholesterol (-30%) and higher triglycerides (+48%) in men but not in women as compared with unaffected family members as well as with controls from the Prospective Cardiovascular Münster (PROCAM) study. Moreover, 11 of 15 male apo A-I(delta K107) heterozygotes but only 2 of 17 female apo A-I(delta K107) heterozygotes had HDL-cholesterol levels below the 20th percentile of sex-matched controls from the PROCAM study. We conclude that heterozygosity for apo A-I(delta K107) decreases HDL-cholesterol and increases triglycerides in men but not in women.

Synthetic model peptides for apolipoproteins. I. Design and properties of synthetic model peptides for the amphipathic helices of the plasma apolipoproteins

Amphipathic helical peptides are the lipid-binding motives of the plasma apolipoproteins, and synthetic peptide analogs have been used to unravel the mechanism of lipid association within this class of proteins. Hydrophobic interactions between the apolar amino acid residues belonging to the hydrophobic face of the amphipathic helices and the lipids are the major driving forces in the peptide-lipid association to form discoidal complexes. Ionic interactions and salt bridge formation between contiguous peptide chains in the complex can, however, contribute to the overall stability of the lipid-protein particle. This was studied by designing peptide analogs to the helical repeats of the apolipoproteins with variable degrees of salt bridge formation between adjacent peptide chains. The most stable conformation for pairs of synthetic peptides was calculated by energy minimisation together with the energy of interaction between peptides. The sequence of the peptides was derived from that of the 18A peptide synthesized by Segrest et al., and the theoretical calculations confirmed that ionic interactions between residues close to each other, along the edge of two adjacent anti-parallel peptides, can significantly contribute towards the stability of a peptide-phospholipid complex.

Conformational analysis of apolipoprotein A-I and E-3 based on primary sequence and circular dichroism

The primary and secondary structure of human plasma apolipoprotein A-I and apolipoprotein E-3 have been analyzed to further our understanding of the secondary and tertiary conformation of these proteins and the structure and function of plasma lipoprotein particles. The methods used to analyze the primary sequence of these proteins used computer programs: (a) to identify repeated patterns within these proteins on the basis of conservative substitutions and similarities within the physicochemical properties of each residue; (b) for local averaging, hydrophobic moment, and Fourier analysis of the physicochemical properties; and (c) for secondary structure prediction of each protein carried out using homology, statistical, and information theory based methods. Circular dichroism was used to study purified lipid-protein complexes of each protein and quantitate the secondary structure in a lipid environment. The data from these analyses were integrated into a single secondary structure prediction to derive a model of each protein. The sequence homology within apolipoproteins A-I, E-3, and A-IV is used to derive a consensus sequence for two 11 amino acid repeating sequences in this family of proteins.

LCAT activation properties of apo A-I CNBr fragments and conversion of discoidal complexes into spherical particles

We studied the substrate properties of the phospholipid-cholesterol-apolipoprotein complexes generated with apo A-I, apo A-I-CNBr fragments, apo A-II and apo A-IV for cholesterol esterification by the enzyme lecithin-cholesterol acyltransferase (LCAT). The kinetic parameters determined with the different complexes as substrates, showed that the complexes containing apo A-I and apo A-IV were about 40-times more efficient than those generated with the apo A-I fragments. In this system, the substrates containing apo A-II had the lowest efficiency. In spite of the differences in the kinetic parameters observed with the various apolipoprotein-lipid complexes, the cholesterol inserted in the complexes was esterified for more than 90% after 24 h in all systems studied. Based upon the results of the kinetic experiments, we followed the transformation of the discoidal complexes into spherical particles, due to the formation of a cholesteryl esters core, in the presence of low-density lipoproteins as an external source of cholesterol. We observed the formation of spherical particles by electron microscopy, after incubation of the discoidal complexes with LCAT for 24 h. The average percentage of cholesteryl esters in the converted particles was around 60% of the total cholesterol, varying between 40% for the apo A-I-CNBr-1-DPPC-cholesterol complex and up to 86% for the apo A-I-DPPC-cholesterol complex. The secondary structure of protein in the complexes was not significantly modified. However, the phospholipid phase transition disappeared, together with the parallel orientation of the phospholipid acyl chains with the helical segments of the apolipoproteins, as the phospholipids are organized in a monolayer at the surface of the spheres.

Molecular modeling of the amphipathic helices of the plasma apolipoproteins

In this paper we propose a classification of the amphipathic helical repeats occurring in the plasma apolipoprotein sequences. It is based upon the calculation of the molecular hydrophobicity potential around the helical segments. The repeats were identified using a new autocorrelation matrix, based upon similarities of hydrophobic and hydrophilic properties of the amino acid residues within the apolipoprotein sequences. The helices were constructed by molecular modeling, the molecular hydrophobicity potential was calculated, and isopotential contour lines drawn around the helices yielded a three-dimensional visualization of the hydrophobicity potential. Two classes of apolipoproteins could be differentiated by comparing the hydrophobic angles obtained by projection of the isopotential contour lines on a plane perpendicular to the long axis of the helix. The isopotential contour lines around apo AI, AIV, and E are more hydrophilic than hydrophobic, whereas they are of similar intensity for apo AII, CI, and CIII. In both cases discoidal lipid-protein complexes are generated, with the amphipathic helices around the edge of the lipid core. The long axis of the helices is oriented parallel to the phospholipid acyl chains and the hydrophilic side of the helix toward the aqueous phase. As a result of the differences in hydrophobicity potential, the contact between the hydrophobic side of the helices and the phospholipid acyl chains is larger for apo AII, CI, and CIII than for the other apolipoproteins. This might account for the greater stability of the discoidal complexes generated between phospholipids and these apoproteins.

Sequence specific 1H-NMR assignments and secondary structure of a carboxy-terminal functional fragment of apolipoprotein CII

The structural properties of a synthetic fragment of human apolipoprotein CII (apoCII) has been studied by circular dichroism and proton nuclear magnetic resonance. The fragment corresponds to the carboxy-terminal 30 amino acid residues and retains the ability of apoCII to activate lipoprotein lipase. Like native apoCII, the fragment has a tendency to self-associate in pure aqueous solution. Addition of 1,1,1,3,3,3-hexafluoro-2-isopropanol to aqueous solvent dissolves the aggregates and leads to an increase in the alpha-helical content of the peptide, probably by stabilizing transient helical structures. The resonances in the 1H-NMR spectrum of the fragment in 35% (CF3)2CHOH were assigned through standard procedures from nuclear Overhauser enhancement spectroscopy, correlated spectroscopy and total correlated spectroscopy experiments. The NMR data indicates the formation of a stable alpha helix spanning Ile66-Gly77. Another alpha helical turn may be formed between Lys55 and Ala59 and possibly span even further towards the carboxyl terminus. These structural elements are different from those previously predicted for this part of the sequence of apoCII.

Orientation into the lipid bilayer of an asymmetric amphipathic helical peptide located at the N-terminus of viral fusion proteins

The complete amino-acid sequence of viral fusion proteins has been analyzed by the Eisenberg procedure. The region surrounding the cleavage site contains a highly hydrophilic region immediately followed by a membrane-like region. Since the effective cleavage between these two domains seems required to expose the fusogenic domain (located at the N-terminal sequence of the transmembrane like region) which is assumed to interact with the lipid membrane of the host cell, we have focused our analysis on the conformation and mode of insertion of this membrane-like domain in a lipid monolayer. It was inserted as an alpha-helical structure into a dipalmitoylphosphatidylcholine (DPPC) monolayer and its orientation at the lipid/water interface was determined using a theoretical analysis procedure allowing the assembly of membrane components. For each viral protein sequence these N-terminal helical segments oriented obliquely with respect to the lipid/water interface. This rather unusual orientation is envisaged as a prerequisite to membrane destabilization and fusogenic activity.

Apolipoprotein A-I variants. Naturally occurring substitutions of proline residues affect plasma concentration of apolipoprotein A-I

Six unrelated families with genetically determined structural variants of apo A-I were found in the course of an electrophoretic screening program for apo A-I variants in dried blood samples of newborns. The following structural variations were identified by the combined use of HPLC, time-of-flight secondary ion mass spectrometry (TOF-SIMS), and automated gas phase sequencing: Pro3----Arg (1x), Pro4----Arg (1x), and Pro165----Arg (4x). All variant carriers were heterozygous for their mutant of apo A-I. Subjects heterozygous for apo A-I(Pro165----Arg) (n = 12) were found to exhibit lower mean values for apo A-I (109 +/- 16 mg/dl) and HDL cholesterol (37 +/- 9 mg/dl) than unaffected family members (n = 9): 176 +/- 41 and 64 +/- 18 mg/dl, respectively (P less than 0.001). In 9 of 12 apo A-I(Pro165----Arg) variant carriers the concentrations of apo A-I were below the fifth percentile of sex-matched controls. By two-dimensional immunoelectrophoresis as well as by densitometry the relative concentration of the variant apo A-I in heterozygous carriers of apo A-I(Pro165----Arg) was determined to account for only 30% of the total plasma apo A-I mass instead of the expected 50%. Thus, the observed apo A-I deficiency may be largely a consequence of the decreased concentration of the variant apo A-I. In the case of the apo A-I(Pro3----Arg) mutant, densitometry of HDL apolipoproteins demonstrated a distinctly increased concentration of the variant proapo A-I relative to normal proapo A-I. This phenomenon was not observed in the apo A-I(Pro4----Arg) mutant or in other mutants. This suggests that the interspecies conserved proline residue in position 3 of mature apo A-I is functionally important for the regular enzymatic conversion of proapo A-I to mature apo A-I.

Topo-dynamic characteristics of human plasma VLDL apolipoproteins and efficiency of triacylglycerol hydrolysis by lipoprotein lipase

A lower accessibility to water-soluble quenchers of tryptophanyls of VLDL apolipoproteins B, E, C as compared to LDL apoB chromophores has been detected by a fluorescence quenching technique. The dynamic behaviour of the tryptophanyls of VLDL amphipathic apolipoproteins E and C did not change in the presence of a detergent, Tween-20, at sub-lytic concentrations. However, a reversible structural transition registered by the 'red' shift of the emission spectrum maximum and the changes in the quenching pattern by I- occurred under these conditions. The increase in the VLDL tryptophanyl accessibility to acrylamide and the decrease in the quenching constant were observed at partial and complete solubilization of the VLDL particles by the detergent. Dissociation of apolipoproteins from VLDL occurred after their treatment with Tween-20 or lipoprotein lipase isolated from bovine milk, and the tryptophanyl population not participating in fluorescence energy transfer on lipid phase-localized fluorescent probe pyrene appeared. In the presence of Tween-20, the relative affinity of apoE for the lipid matrix of VLDL was lower than that of apoC. Besides, the uncompetitive mode of inhibition of the LPL activity by apoC-III has been demonstrated. It is suggested that: (1) the amphipathic apolipoproteins E and C are organized as clusters on the VLDL surface and/or partially shielded by apolipoprotein B: (2) self-regulation of lypolysis may exist involving detergent-like reaction product accumulation and changes in relative apolipoprotein contents.

Identification of peptide hormones of the amphipathic helix class using the helical hydrophobic moment algorithm

Eisenberg's helical hydrophobic moment (less than mu H greater than) algorithm was applied to the analysis of the primary structure of amphipathic alpha-helical peptide hormones and an optimal method for identifying other peptides of this class determined. We quantitate and compare known amphipathic helical peptide hormones with a second group of peptides with proven nonamphipathic properties and determine the best method of distinguishing between them. The respective means of the maximum 11 residue less than mu H greater than for the amphipathic helical and control peptides were 0.46 (+/-/-0.07) and 0.33 (0.07) (P + 0.004). To better reflect the amphipathic potential of the entire peptide, the percent of 11 residue segments in each peptide above a particular less than mu H greater than was plotted vs less than mu H greater than. The resulting curves are referred to as HM-C. The mean HM-C (of the two groups) was highly significantly different such that the HM-C method was superior to others in its ability to distinguish amphipathic from nonamphipathic peptides. Several potential new members of this structural class were identified using this approach. Molecular modeling of a portion of one of these, prolactin inhibitory factor, reveals a strongly amphipathic alpha helix at residues 4-21. This computer-based method may enable rapid identification of peptides of the amphipathic alpha-helix class.

Conformational analysis of lipid-associating proteins in a lipid environment

Two major types of helical structures have been identified in lipid-associating proteins, being either amphipathic or transmembrane domains. A conformational analysis was carried out to characterize some of the properties of these helices. These calculations were performed both on isolated helices and in a lipid environment. According to the results of this analysis, the orientation of the line joining the hydrophobic and hydrophilic centers of the helix seems to determine the orientation of the helix at the lipid/water interface. The calculation of this parameter should be useful to discriminate between an amphipathic helix, parallel to the interface and a transmembrane helix orientated perpendicularly. The membrane-spanning helices are completely immersed in the phospholipid bilayer and their length corresponds to about the thickness of the hydrophobic core of the DPPC bilayer. The energy of interaction, expressed per phospholipid is significantly higher for the transmembrane compared to the amphipathic helices. For the membrane-spanning helices the mean energy of interaction is higher than the interaction energy between two phospholipids, while it is lower for most amphipathic helices. This might account for the stability of these protein-anchoring domains. This computer modeling approach should usefully complement the statistical analysis carried out on these helices, based on their hydrophobicity and hydrophobic moment. It represents a more refined analysis of the domains identified by the prediction techniques and stress the functional character of lipid-associating domains in membrane proteins as well as in soluble plasma lipoproteins.

Immunochemical characterization of two antigenic sites on human apolipoprotein A-I; localization and lipid modulation of these epitopes

Two monoclonal antibodies, A17 and A30, were raised against human apolipoprotein A-I (apo A-I). They were studied by competitive inhibition of 125I-labeled HDL3 with HDL subfractions, delipidated apo A-I, and complexes of dimyristoylphosphatidylcholine (DMPC) containing apo A-I and apo A-II. Immunoblotting located the A17 antibody on CNBr fragment 4 of apo A-I and the A30 antibody on CNBr fragment 1. The A17 antigenic determinant was expressed identically in all HDL subclasses, on delipidated apo A-I as well as all on the DMPC-apo A-I and DMPC-apo A-I/apo A-II complexes. In contrast, the apparent affinity constant of the A30 antibody for delipidated apo A-I was about 30-times less than for HDL3 or for apo A-I/apo A-II-phospholipid complexes. These data suggest that the association of apo A-I with phospholipids improves the reactivity of the A30 monoclonal antibody towards apo A-I, and that this antigenic determinant has a different conformation in delipidated apo A-I compared to apo A-I complexed with phospholipids. Turbidimetric and fluorescence experiments monitoring the phospholipid-apo A-I association in the presence and in the absence of the A17 and A30 antibodies were consistent with the competition experiments carried out by solid phase radioimmunoassay (RIA). After reaction of apo A-I with the A30 antibody, we observed an enhancement of the degradation kinetics of large multilamellar vesicles (LMV), while the A17 antibody did not have a significant effect. Calcein leakage experiments carried out below the transition temperature of DPPC showed an enhancement of the degradation kinetics with both monoclonal antibodies, while the phase-transition release was independent of the reaction of apo A-I with the monoclonal antibodies. These data therefore suggest the existence of at least two different types of epitope on apo A-I, which might account for the differences in immunological reactivity of apo A-I that is either delipidated or present on HDL.

Reduction in antiviral activity of human interferon-gamma in acidic media with reference to structural change

The fluorescence intensity of a unique tryptophan 36 in human interferon-gamma was drastically decreased below pH 4 with a concomitant decrease of antiviral activity. The region of residues 32-42 of human interferon-gamma was found by calculation to have a low hydrophobicity together with a high helical hydrophobic moment, and the net electric charge of this region having an amphiphilic helical structure changed significantly near pH 4. These results suggest that the region of residues 32-42 plays an important role in exhibiting antiviral activity.