Phospholipid membrane stabilization by dimethylsulfoxide and other inducers of Friend leukemic cell differentiation

A large number of low molecular weight polar cryoprotective agents have recently been found to induce erythroid differentiation of Friend leukemic cells in vitro. The effect of these agents on membrane fluidity in phospholipid vesicles was studied by determining the solid-to-liquid crystalline phase transition using differential scanning calorimetry. Some of the inducing agents studies were found to raise the normal transition temperature (Tc) by a few degrees. All of these agents were found to produce a separate transition at a much higher temperature. Changes in the head group of the phospholipid, the pH, the presence of divalent cations, and the addition of other membrane-active compounds were found to significantly influence the inducing agent's effects on the Tc of phospholipid membranes. The ability of the different agents to produce a new transition at a high temperature was found to correlate well with their ability to induce Friend leukemic cell differentiation. The possible mechansims of action of the chemical inducers, and the significance of the observed membrane effects on differentiation and malignancy are discussed. It is concluded that inducing agents decrease the fluidity and stabilize phospholipid membranes, and that their effects in cell differentiation might be initiated by a similar change in the properties of cell membranes.     

The nucleation and growth of calcium phosphate crystals at protein and phosphatidylserine liposome surfaces

The kinetics of calcium phosphate crystal growth at the surfaces of proteins and phospholipids has been investigated using free drift and constant composition methods in supersaturated calcium phosphate solutions (relative supersaturations: with respect to hydroxyapatite, HAP, sigma HAP = 15.0, and with respect to octacalcium phosphate, OCP, sigma OCP = 1.9). Fibrinogen and collagen molecules adsorbed at hydrophobic surfaces as well as uncross-linked collagen fibrils induce ion binding and subsequent nucleation of calcium phosphate. The formation of OCP on phosphatidylserine vesicles introduced to highly supersaturated calcium phosphate solutions probably involves the interaction of the calcium ions with the ionized carboxylic groups of the phospholipid.     

An analysis of simultaneous variation in protein structures

The simultaneous substitution of pairs of buried amino acid side chains during divergent evolution has been examined in a set of protein families with known crystal structures. A weak signal is found that shows that amino acid pairs near in space in the folded structure preferentially undergo substitution in a compensatory way. Three different physicochemical types of covariation 'signals' were then examined separately, with consideration given to the evolutionary distance at which different types of compensation occur. Where the compensatory covariation tends towards retaining the combined residue volumes, the signal is significant only at very low evolutionary distances. Where the covariation compensates for changes in the hydrogen bonding, the signal is strongest at intermediate evolutionary distances. Covariations that compensate for charge variations appeared with equal strength at all the evolutionary distances examined. A recipe is suggested for using the weak covariation signal to assemble the predicted secondary structural elements, where the evolutionary distance, covariation type and weighting are considered together with the tertiary structural context (interior or surface) of the residues being examined.      

Interaction of non-histone chromosomal proteins HMG1 and HMG2 with DNA

Interaction between non-histone protein HMG1 or HMG2 and DNA has been studied by using thermal denaturation and circular dichroism (CD) spectroscopy. We have made the following observations. 1. The binding of each of these two proteins to DNA stabilizes the latter, as shown by an increase in melting temperature of 20 degrees C (from 45 degrees C to about 65 degrees C). 2. There are 6.0 amino acids/nucleotide in HMG1-bound DNA and 5.0 in HMGI-bound DNA which suggests that each HMB1 moleculae would cover about 20 base pairs of DNA and each HMG2 molecule would cover about 25 base pairs. 3. The alpha-helical content of these two non-histone proteins in the complexes, estimated from the CD value at 220 nm, is about one third to one half that of total proteins in calf thymus chromatin. 4. DNA conformation is distorted only slightly by the binding of protein HMG1 or HMG2. 5. Neither the melting nor the CD properties of HMG1-DNA or HMG2-DNA complexes differ substantially whether they are prepared by NaCl-gradient dialysis in urea or by direct mixing of protein and DNA at 0.15 M NaCl, followed by dialysis against the same buffer i.e. 0.25 mM EDTA (pH 8.0).     

Synthesis and in vitro antimalarial activity of sulfone endoperoxides

A series of 4,8-dimethyl-4-phenylsulfonylmethyl-2,3-dioxabicyclo[3.3.1]+ ++nonanes, carrying a variety of substituents at position-8 (4) were prepared by a short and efficient method from R-(+)-limonene. Key reactions include thiol oxygen cooxidation, and alkylation and acylation of a sterically hindered tertiary alcohol compatible with the endoperoxy functionality. Some of compounds 4, which are structurally related to yingzhaosu A (2), were found to exhibit in vitro antimalarial activity comparable to that of artemisinin (1) and superior to that of arteflene (3).