Helical peptide models for protein glycation: proximity effects in catalysis of the Amadori rearrangement

INTRODUCTION

Non-enzymatic glycation of proteins has been implicated in various diabetic complications and age-related disorders. Proteins undergo glycation at the N-terminus or at the epsilon-amino group of lysine residues. The observation that only a fraction of all lysine residues undergo glycation indicates the role of the immediate chemical environment in the glycation reaction. Here we have constructed helical peptide models, which juxtapose lysine with potentially catalytic residues in order to probe their roles in the individual steps of the glycation reaction.

RESULTS

The peptides investigated in this study are constrained to adopt helical conformations allowing residues in the i and i+4 positions to come into spatial proximity, while residues i and i+2 are far apart. The placing of aspartic acid and histidine residues at interacting positions with lysine modulates the steps involved in early peptide glycation (reversible Schiff base formation and its subsequent irreversible conversion to a ketoamine product, the Amadori rearrangement). Proximal positioning of aspartic acid or histidine with respect to the reactive lysine residue retards initial Schiff base formation. On the contrary, aspartic acid promotes catalysis of the Amadori rearrangement. Presence of the strongly basic residue arginine proximate to lysine favorably affects the pK(a) of both the lysine epsilon-amino group and the singly glycated lysine, aiding in the formation of doubly glycated species. The Amadori product also formed carboxymethyl lysine, an advanced glycation endproduct (AGE), in a time-dependent manner.

CONCLUSIONS

Stereochemically defined peptide scaffolds are convenient tools for studying near neighbor effects on the reactivity of functional amino acid sidechains. The present study utilizes stereochemically defined peptide helices to effectively demonstrate that aspartic acid is an efficient catalytic residue in the Amadori arrangement. The results emphasize the structural determinants of Schiff base and Amadori product formation in the final accumulation of glycated peptides.

Fluoroalcohols as structure modifiers in peptides and proteins: hexafluoroacetone hydrate stabilizes a helical conformation of melittin at low pH

The effect of hexafluoroacetone hydrate (HFA) on the structure of the honey bee venom peptide melittin has been investigated. In aqueous solution at low pH melittin is predominantly unstructured. Addition of HFA at pH approximately 2.0 induces a structural transition from the unstructured state to a predominantly helical conformation as suggested by intense diagnostic far UV CD bands. The structural transition is highly cooperative and complete at 3.6 M (50% v/v) HFA. A similar structural transition is also observed in 2,2,2 trifluoroethanol which is complete only at a cosolvent concentration of approximately 8 M. Temperature dependent CD experiments support a 'cold denaturation' of melittin at low concentrations of HFA, suggesting that selective solvation of peptide by HFA is mediated by hydrophobic interactions. NMR studies in 3.6 M HFA establish a well-defined helical structure of melittin at low pH, as suggested by the presence of strong NH/NHi+1 NOEs throughout the sequence, along with many medium range helical NOEs. Structure calculations using NOE-driven distance constraints reveal a well-ordered helical fold with a relatively flexible segment around residues T10-G11-T12. The helical structure of melittin obtained at 3.6 M HFA at low pH is similar to those determined in methanolic solution and perdeuterated dodecylphosphocholine micelles. HFA as a cosolvent facilitates helix formation even in the highly charged C-terminal segment.

X-ray Studies on Crystalline Complexes Involving Amino Acids and Peptides. XXXIII. Crystal Structures of L- and DL-Arginine Complexed with Oxalic Acid and a Comparative Study of Amino Acid–Oxalic Acid Complexes

The DL- and L-arginine complexes of oxalic acid are made up of zwitterionic positively charged amino acid molecules and semi-oxalate ions. The dissimilar molecules aggregate into separate alternating layers in the former. The basic unit in the arginine layer is a centrosymmetric dimer, while the semi-oxalate ions form hydrogen-bonded strings in their layer. In the L-arginine complex each semi-oxalate ion is surrounded by arginine molecules and the complex can be described as an inclusion compound. The oxalic acid complexes of basic amino acids exhibit a variety of ionization states and stoichiometry. They illustrate the effect of aggregation and chirality on ionization state and stoichiometry, and that of molecular properties on aggregation. The semi-oxalate/oxalate ions tend to be planar, but large departures from planarity are possible. The amino acid aggregation in the different oxalic acid complexes do not resemble one another significantly, but the aggregation of a particular amino acid in its oxalic acid complex tends to have similarities with its aggregation in other structures. Also, semi-oxalate ions aggregate into similar strings in four of the six oxalic acid complexes. Thus, the intrinsic aggregation propensities of individual molecules tend to be retained in the complexes.

X-ray studies on crystalline complexes involving amino acids and peptides. XXXII. Effect of chirality on ionisation state, stoichiometry and aggregation in the complexes of oxalic acid with DL- and L-lysine

Crystals of the oxalic acid complex of DL-lysine (triclinic P1; a = 5.540(1), b = 10.764(2), c = 12.056(2) A, alpha = 77.8(1), beta = 80.6(1), gamma = 75.6(1).; R = 4.7% for 2023 observed reflections) contain lysine and semioxalate ions in the 1:1 ratio, whereas the ratio of lysine and semioxalate/oxalate ions is 2:3 in the crystals of the L-lysine complex (monoclinic P2(1); alpha = 4.906(1), b = 20.145(4), c = 12.455(1) A, beta = 92.5(1).; R = 4.4% for 1494 observed reflections). The amino acid molecule in the L-lysine complex has an unusual ionisation state with positively charged alpha- and side-chain amino groups and a neutral carboxyl group. The unlike molecules aggregate into separate alternating layers in the DL-lysine complex in a manner similar to that observed in several of the amino acid complexes. The L-lysine complex exhibits a new aggregation pattern which cannot be easily explained in terms of planar features, thus emphasizing the fundamental dependence of aggregation on molecular characteristics. Despite the differences in stoichiometry, ionisation state and long-range aggregation patterns, the basic element of aggregation in the two complexes exhibits considerable similarity.

Insulin binding to liver nuclei from lean and obese mice is altered by dietary fat

Insulin binding to the plasma membrane is known to be altered by modifying the membrane composition through dietary treatment. As insulin binding receptors are also present on nuclear membrane, this study was undertaken to investigate if specific binding of insulin to the liver nuclei is altered by diet. 8-wk-old female C57 B 6J lean and ob/ob mice were fed semipurified diets containing 20% (w/w) fat of either high or low polyunsaturated-to-saturated (P/S) fatty acid ratio for 4 wk. Liver nuclei were prepared, insulin binding was measured and nuclear phospholipids were isolated for lipid analysis. Insulin binding was highest in nuclei prepared from lean mice fed a high P/S diet. Specific binding of insulin to nuclei prepared from obese mice was also increased by the high P/S diet, but to a lesser extent compared to lean mice. Feeding a high P/S diet increased polyunsaturated fatty acid content of membrane phospholipids from both lean and ob/ob mice. Obese mice were characterized by higher levels of arachidonic acid and lower levels of linoleic acid in phosphatidylcholine. The present study establishes that insulin binding to liver nuclei is increased by feeding a high P/S diet, and that insulin binding to liver nuclei from obese mice is lower than from lean mice.

Modulation of age-related alterations in membrane composition and receptor-associated immune functions by food restriction in Fischer 344 rats

Food restriction is known to modulate aging and age-associated immune functions in rodents. In an attempt to understand the mechanism(s) through which food restriction delays age-associated loss of certain immune functions, lipid composition of spleen cells as well as binding of spleen cells to interleukin-2 (IL-2) and insulin were analyzed in four month-old and 19-month-old ad libitum fed (AL) and food-restricted (FR) Fischer-344 male rats. The results revealed that although AL-fed rats did not show a difference in age-related changes for IL-2 and insulin binding, the number of binding sites were significantly increased in the spleen cells of 19-month-old FR animals when compared with those of the 19-month-old AL group. When spleen cell phospholipid fractions were analyzed for fatty acid composition, the spleen cells from FR animals consistently revealed higher linoleic acid (18:2) levels and significantly lower arachidonic acid (20:4) and long chain fatty acid, docosatetraenoic acid (22:4) levels in the phosphatidylcholine and phosphatidylethanolamine fractions than the spleen cells of the AL rats. Further, spleen cell plasma membranes of FR rats also exhibited similar changes showing higher 18:2 and lower 20:4 and 22:4 levels than the AL animals. Finally, spleen cells obtained from 19-month-old FR animals produced higher levels of IL-2 and lesser prostaglandin E2 when compared to 19-month-old AL animals. The above observations suggest that one of the mechanisms through which food restriction may delay the loss of age-associated immune functions is through modulation of the fatty acyl composition of phospholipid fractions of spleen cell membranes. This modification may facilitate binding of IL-2 and insulin to their receptors and thus may improve T cell proliferation and prevent or delay age-related loss in immune functions.

The effect of elevated levels of thromboxane on host response to tumor

Previous studies have demonstrated that human malignancies can synthesize large amounts of thromboxane. It has also been reported that thromboxane can significantly alter multiple components of physiologic and immunologic function. We investigated the effect of elevated levels of thromboxane on host response to tumor using multiple rat models, and the long acting thromboxane analogue U-46619. Administration of the thromboxane analogue was not found to significantly alter the growth of primary tumors or peritoneal metastases. The analogue was found to significantly decrease mean survival time with a pulmonary metastases model. The thromboxane analogue failed to alter macrophage cytotoxicity, lymphocyte cytotoxicity, T lymphocyte subset numbers, or lymphocyte blastogenic response. Administration of the thromboxane analogue decreased the rate of lymphocyte metabolism of glucose and decreased lymphocyte intracellular adenosine deaminase activity. In conclusion, elevated thromboxane levels do not appear to alter primary tumor growth or host immune function, but do decrease resistance to pulmonary metastases.

Effect of prostaglandin E in multiple experimental models. VIII. Effect on host response to metastatic tumor

Prostaglandin E (PGE) is produced by certain tumors and is reported to decrease primary tumor growth. We evaluated its effect in multiple tumor models utilizing a 1 week course of the long acting PGE derivative dimethyl-PGE (dPGE) at a dosage of 100 micrograms/kg/day vs. a lactated Ringers control. For all tumor models, a suspension of 1 x 10(6) colon carcinoma cells were injected into Wistar-Furth rats. When the suspension was injected subcutaneously and the drug was begun at the time of tumor challenge, there was no effect on survival. When the tumor was injected intraperitoneally or intravenously and the drug begun at the time of tumor challenge, dPGE decreased survival time. When the tumor was administered intravenously but dPGE was delayed for 5 days, there was no effect on survival time. When rats were given a 1 week course of dPGE or saline, dPGE was found not to alter natural killer (NK) cell cytotoxicity, macrophage cytotoxicity, spontaneous lymphocyte blastogenesis, or mitogen stimulated blastogenesis. dPGE failed to alter lymphocyte metabolism of glucose in nonstimulated lymphocytes, but decreased the rate of glucose metabolism and adenosine deaminase activity in mitogen stimulated lymphocytes. In conclusion, PGE appears to enhance metastatic growth of tumor lines where it does not alter primary tumor growth. This effect does not appear immunologically mediated.

Dietary fat: exogenous determination of membrane structure and cell function

Evidence indicates that principal features of the membrane involve structural organization of lipids in the form of a bilayer with functional proteins either bound to the bilayer surface or inserted into the bilayer and interacting within specific domains in the lipid milieux. In homeotherms, intrinsic and extrinsic factors apparently form the basis for determination of membrane lipid composition and thus membrane physicochemical properties. Moreover, many intrinsic metabolic controls, such as fatty acid desaturation and phospholipid biosynthesis, may be attenuated by change in the nature of the extrinsic or dietary influence. This review will focus on the role of dietary fat as a determinant of subcellular structural constituents to illustrate that feeding nutritionally adequate diets differing in fatty acid composition can induce physiological transitions in membrane function involving the activity of enzymes responsible for synthesis of membrane constituents, hormone-activated functions and expression of activity in the cell nucleus.

Modulation of gene expression in autoimmune disease and aging by food restriction and dietary lipids

Several recent observations carried out by many investigators have offered some clues in understanding the mechanism of how food restriction (FR) acts in the prolongation of life-span, but the precise mechanisms involved in modulating the immune system have not been clearly understood. Our own ongoing studies indicate that FR may act at the molecular level and may extend the life-span by modulating functional activities of several genes in various target tissues. For instance, while cytochrome P-450 IIB1 and IIB2 expression is known to decline with age in ad libitum-fed rats, FR prevented the loss of (drug-inducible) P-450 enzymes in liver tissues. In addition, both alpha 2u-globulin and senescence marker protein 2 expressions, which are regulated by hormones, were also modulated during aging by FR in Fischer 344 male rats. In short-lived autoimmune-prone mice, both FR and omega-3 (n-3) fatty acids diet lowered the severity of autoimmune disease both in lupus-prone (NZB x NZW)F1 mice and in mice prone to develop lymphoproliferative and renal diseases, whereas saturated (n-9) and polyunsaturated (n-6) dietary lipids not only exacerbated autoimmune disease, but also significantly enhanced expression of several oncogenes in lymphoid tissues. FR and omega-3 fatty acids decreased the expression of certain oncogenes. Both FR and omega-3 fatty acids may modulate the aging and autoimmune disease processes by not only altering the fatty acid composition, membrane fluidity, and signal transduction, but also by modulating the lymphokine hormone receptors and their functions and thereby modulating expression of several genes in various tissues during the aging process.