Phase partitioning detects differences between phospholipase-released forms of alkaline phosphatase--a GPI-linked protein

Aqueous Two-Phase Systems and Microfluidics for Microscale Assays and Analytical Measurements

Phase separation is a common occurrence in nature. Synthetic and natural polymers, salts, ionic liquids, surfactants, and biomacromolecules phase separate in water, resulting in an aqueous two-phase system (ATPS). This review discusses the properties, handling, and uses of ATPSs. These systems have been used for protein, nucleic acid, virus, and cell purification and have in recent years found new uses for small organics, polysaccharides, extracellular vesicles, and biopharmaceuticals. Analytical biochemistry applications such as quantifying protein-protein binding, probing for conformational changes, or monitoring enzyme activity have been performed with ATPSs. Not only are ATPSs biocompatible, they also retain their properties at the microscale, enabling miniaturization experiments such as droplet microfluidics, bacterial quorum sensing, multiplexed and point-of-care immunoassays, and cell patterning. ATPSs include coacervates and may find wider interest in the context of intracellular phase separation and origin of life. Recent advances in fundamental understanding and in commercial application are also considered.

Effects of osmolytes on protein–solvent interactions in crowded environments: study of sucrose and trehalose effects on different proteins by solvent interaction analysis

The logarithms of the partition coefficients of proteins in the presence of 0.5 M sucrose and trehalose are linearly interrelated.

Structural features important for differences in protein partitioning in aqueous dextran-polyethylene glycol two-phase systems of different ionic compositions

Partitioning of 15 proteins in dextran-70-polyethylene glycol (PEG)-8000 aqueous two-phase systems (ATPSs) in the presence of 0.01M sodium phosphate buffer, pH7.4 was studied. The effect of salt additives (NaCl, CsCl, Na2SO4, NaClO4 and NaSCN) at different concentrations on the protein partition behavior was examined. The salt effects on protein partitioning were analyzed by using the Collander solvent regression relationship between the protein partition coefficients in ATPSs with and without salt additives. The results obtained show that the presence and concentration of salt additives affect the protein partition behavior. Analysis of ATPSs in terms of the differences between the relative hydrophobicity and electrostatic properties of the phases does not explain the protein partition behavior. The differences between protein partitioning could not be explained by the protein size. The structural signatures for the proteins were constructed from partition coefficient values in four ATPSs with different salt additives, and the structural distances were calculated using cytochrome c as the reference structure. The structural distances for all the examined proteins (except lysozyme) were found to be interrelated. Analysis of about 50 different descriptors of the protein structures revealed that the partition behavior of proteins is determined by the peculiarities of their surfaces (e.g., the number of water-filled cavities and the averaged hydrophobicity of the surface residues) and by the intrinsic flexibility of the protein structure measured in terms of the B-factor (or temperature factor).

Human tissue non-specific alkaline phosphatases: sugar-moiety-induced enzymic and antigenic modulations and genetic aspects

To investigate the possible role(s) of glycans in human tissue non-specific alkaline phosphatase (TNAP) activity, the iso-enzymes were purified and treated with various exo- and endo-glycosidases. Catalytic activity, oligomerization, conformation and immunoreactivity of the modified TNAPs were evaluated. All TNAPs proved to be N-glycosylated, and only the liver isoform (LAP) is not O-glycosylated. Usually, the kidney (KAP) and bone (BAP) isoenzymes are similar and cannot be clearly discriminated. Differences between the immunoreactivity of KAP/BAP and LAP with a BAP antibody were mainly attributed to the N-glycosylated moieties of the TNAPs. In addition, elimination of O-glycosylations moderately affects the TNAP reactivity. Interestingly, N-glycosylation is absolutely essential for TNAP activity, but not for that of the placental or intestinal enzymes. According to the deduced amino acid sequence of TNAP cDNA, Asn-213 is a possible N-glycosylation site, and our present findings suggest that this sugar chain plays a key role in enzyme regulation. With regard to the oligomeric state of alkaline phosphatase (AP) isoforms, the dimer/tetramer equilibrium is dependent on the deglycosylation of glycosyl-phosphatidylinositol(GPI)-free APs, but not GPI-linked APs. This equilibrium does not affect the AP conformation as observed with CD. With regard to TNAPs, no data were available on the gene expression or nature of the 5'-non-translated leader exon of human KAP, as opposed to BAP and LAP genes. cDNA sequencing revealed that cortex/medulla KAP is genetically related to BAP, and medulla KAP to LAP.

Physicochemical and pathophysiological factors in the release of membrane-bound alkaline phosphatase from cells

Alkaline phosphatase is bound to cell membranes by a glycan phosphatidylinositol anchoring domain. The structure of this domain and ways in which it may be cleaved by chemical and enzymatic means provide a basis for understanding the solubilization of alkaline phosphatase from tissues in vitro and in vivo and the generation of isoforms.

Separation of alkaline phosphatase isoforms with and without intact glycan-phosphatidylinositol anchors in aqueous polymer phase systems

Alkaline phosphatase (ALP) isoforms can be distinguished from each other by their partition characteristics in aqueous two-phase systems composed of water-soluble polymers, the phases of which are differentially sensitive to the presence of glycan-phosphatidylinositol anchors. Compared with detergent-based systems, the aqueous polymer systems have the advantage that micelle formation does not take place. Partition of anchor-intact and anchor-degraded molecules in the latter systems is further improved by attachment of a hydrophobic ligand to one of the phase forming polymers. In this way, anchor-intact molecules can be separated from molecules with degraded anchors in a single partition step. The method has been used to confirm that ALP in human serum is predominantly anchor degraded, whereas in bile it retains its anchor intact.

Release of membrane-bound enzymes from cells and the generation of isoforms

Enzymes bound to the surfaces of cells may be retained by a hydrophobic amino acid sequence (e.g. gamma-glutamyltransferase) or by a specific glycan phosphatidylinositol (GPI) anchor (e.g. alkaline phosphatase). In either case the attachment is by means of non-covalent hydrophobic interactions between the anchoring domain of the enzyme and lipid components of the cell membrane. Enzyme molecules released into the plasma or bile, complete with their hydrophobic domains, can undergo aggregation and complexation to give rise to high molecular weight isoforms of gamma-glutamyltransferase or alkaline phosphatase. However, the GPI domain of alkaline phosphatase can be degraded by an inositol-specific phospholipase in plasma, but not in bile, with production of the hydrophobic, non-aggregating isoform of alkaline phosphatase that predominates in plasma.

A method for the assay of phosphatidylinositol-specific phospholipase D activity in serum

A reproducible substrate for the assay of phosphatidylinositol-specific phospholipase D (PIPLD) can be prepared by extracting alkaline phosphatase from placental tissue with n-butanol under alkaline conditions. The alkaline phosphatase thus prepared retains its hydrophobic glycan phosphatidylinositol (GPI) anchor and aggregates into high M(r) forms. Incubation with serum hydrolyses the phosphate inositol linkage by PIPLD action, producing a less lipophilic, non-aggregated isoform of alkaline phosphatase. Three methods of measuring the amount of this isoform produced after a timed incubation with serum are described and compared: two types of phase partitioning systems, and electrophoresis and densitometry of the products after gradient-pore electrophoresis. All give comparable and reproducible measurements of PIPLD; however, the electrophoretic method is preferred for routine analysis.